Introduction
Duncan (1864) noted that there were ‘paired’ flabellid genera, or maybe only groups of species, differing from one another by only one character, such as the presence or absence of a lamellar columella. He called these ‘mimetic’ genera, or those that mimic one another, except for one character. He also noted that groups of flabellid species might also differ from another by having a pedicellate (attached) base or having transverse division, being otherwise similar. He categorized 10 flabellid species into four permutations or categories (Fig. 1): (1) pedicellate and lacking a columella (typical Flabellum Lesson, Reference Lesson1831, Fig. 1, Category 1), (2) pedicellate but with a lamellar columella (unnamed higher taxon, herein Periplacotrochus, Fig. 1, Category 2), (3) transverse division and lacking a columella (future Truncatoflabellum Cairns, Reference Cairns1989b, Fig. 1, Category 3), and (4) transverse division but with a lamellar columella (typical Placotrochus Milne Edwards and Haime, Reference Milne Edwards and Haime1848, Fig. 1, Category 4). Species in the third category mentioned in the preceding are quite common, represented by at least 38 species, occurring in water depths of 2–3,010 m around the world, as well as in the fossil record as early as the middle Eocene (Cairns, Reference Cairns2016). This group was frequently discussed (see Cairns, Reference Cairns1989b) but not named until 1989, when Cairns (Reference Cairns1989a, Reference Cairnsb) suggested that transverse division conferred an evolutionary advantage to this genus. He called the genus Truncatoflabellum Cairns, Reference Cairns1989b. A similar situation occurs between Category 4 (Placotrochus) and Category two; however, because species belonging to Category two occur exclusively as fossils in southern Australia and are collected, this species group has never been named. Cairns (Reference Cairns1989b, p. 75) had implied the need for such a name, calling this group of species the ‘Placotrochus’ deltoideus species complex (Cairns, Reference Cairns1989b, table 4). The purpose of this paper is to name this genus and redescribe the species contained in it.
Figure 1 The four permutations or categories of genera resulting from the character states of two characters: presence or absence of a lamellar columella and pedicellate vs. transverse division. (Category 1) Flabellum, exemplified by F. magnificum, USNM 81951. (Category 2) Periplacotrochus, exemplified by the holotype of P.cudmorei. (Category 3) Truncatoflabellum, exemplified by the neotype of T. candeanum, USNM 81963. (Category 4) Placotrochus, exemplified by P. laevis, USNM 81994.
According to Duncan (1864), the genus Flabellum could be considered mimetic to Truncatoflabellum, differing only in lacking transverse division. In the same way, Periplacotrochus could be considered mimetic to Flabellum, differing only in having a lamellar columella. Likewise, Truncatoflabellum could be considered mimetic to Placotrochus, differing only in lacking a lamellar columella, and Placotrochus could be considered mimetic to Periplacotrochus, differing only in having transverse division (Fig. 1). Whatever way interpreted, Flabellum is the oldest genus (Late Cretaceous) and could have been the ancestor of Periplacotrochus (earliest known occurrence lower Eocene) and Truncatoflabellum (earliest known occurrence middle Eocene). Placotrochus, known only from the Recent, could have evolved from either Periplacotrochus or Truncatoflabellum by one character state change.
Because of the solitary nature of the corallum and the placement of the genus in the family Flabellidae, species of Periplacotrochus are assumed to have been azooxanthellate. Because of its similarity to Placotrochus and Truncatoflabellum and other associated south Australian Cenozoic corals, especially those found at Muddy Creek and Balcombe’s Bay, Victoria, the species of Periplacotrochus are assumed to have lived in the middle to lower shelf depths, i.e., 50–200 m.
Materials and methods
The terminology used in the descriptions is defined and illustrated in Cairns and Kitahara (Reference Cairns and Kitahara2012). Abbreviations used in the text include the following: Cx, Sx=costae or septa, respectively, of cycle x; GCD=greater calicular diameter; GCD:LCD=ratio of greater calicular diameter to lesser calicular diameter of a corallum; H:GCD=ratio of corallum height to its greater calicular diameter; Sx>Sy=in the context of a septal formula, septa of cycle x are wider than those of cycle y.
Repositories and institutional abbreviations
NHMUK: The Natural History Museum, UK, London; NMNH: National Museum of Natural History, Smithsonian Institution, Washington DC; NMV: National Museum of Victoria, Victoria, Australia; USGS: United States Geological Survey; USNM: United States National Museum (now the NMNH), Washington DC. Many of the specimens reported as new records herein originate from a gift to the Smithsonian from F. A. Cudmore in April 1923.
Systematic paleontology
Class Anthozoa Ehrenberg, Reference Ehrenberg1834
Order Scleractinia Bourne, Reference Bourne1900
Family Flabellidae Bourne, Reference Bourne1905
Diagnosis
Corallum solitary, fixed, free, or transversely dividing. Wall epithecal (neotenic condition), sometimes secondarily thickened with stereome. Epitheca usually smooth, lacking costae. Septa imperforate, composed of a single fan system of numerous closely spaced trabeculae, which produces a smooth axial edge. Pali, dissepiments, and synapticulae absent; paliform lobes present on one genus. Columella usually rudimentary or absent, but may be lamellar, papillose, or fascicular. Exclusively azooxanthellate.
Occurrence
Lower Cretaceous to Recent: worldwide, including off Antarctica, 0–3,186 m.
Remarks
The family was most recently revised by Cairns (Reference Cairns1989b, table 4), who tabularized the characteristics of the various genera. Table 1 is an update and expansion of that compilation, the family now consisting of 14 genera: seven exclusively Recent, four exclusively fossil, and three having both components. 100 Recent species are known in the family (Cairns and Hoeksema, Reference Cairns and Hoeksema2015, table 1), making it the fifth-most species-rich family among the 24 families that contain Recent species in the Scleractinia (Cairns, Reference Cairns1999), and containing about 7% of the living species. Approximately 174 fossil species are known (Table 1); however, the total number of fossil species in the approximately 85 families that contain fossil species is not well known, and thus similar ranking cannot be made for the fossil species.
Table 1 Number of species and distribution of the 14 Recent and fossil flabellid genera.
In the first broadly based phylogenetic analysis of the Scleractinia using molecular sequencing, Kitahara et al. (Reference Kitahara, Cairns, Stolarski, Blair and Miller2010) supported the monophyly of the family Flabellidae but found polyphyletic assemblages among the three multispecies genera they analyzed (i.e., Flabellum, Truncatoflabellum, Javania), implying that the morphological distinctions of these genera may not be accurate. However, Kitahara et al. (Reference Kitahara, Cairns, Stolarski, Blair and Miller2010, p. 4), even though professing to include “species representing the full morphological spectrum of the family,” included species from only four of the 10 Recent genera, and of course none from the four exclusively fossil genera. Furthermore, only one mitochondrial gene was used in the analysis, which is usually not considered to be adequate for phylogenetic analyses. Finally, their conclusion that the transversely dividing genera (e.g., Truncatoflabellum) were antecedent to those with a pedicellate base (e.g., Flabellum) contradicts the fossil record, wherein Flabellum has the oldest fossil occurrence. Thus, based on these three reasons, the morphological distinction of genera is favored in this paper and is the only one that can be used with fossils.
Genus Periplacotrochus new genus
1864 Placotrochus; Reference DuncanDuncan, p. 163, 164, 166–167 (in part).
1884 Placotrochus; Reference DuncanDuncan, p. 16 (in part).
1943 Placotrochus; Reference Vaughan and WellsVaughan and Wells, p. 227 (in part).
1956 Placotrochus; Reference WellsWells, p. F432 (in part).
1989b “Placotrochus” deltoideus species complex; Reference CairnsCairns, p. 44, 45.
Type species
Placotrochus deltoideus Duncan, Reference Duncan1864, here designated.
Diagnosis
Corallum solitary, shaped as a laterally compressed fan; edge spines lacking but may have low thecal crests. Transverse division lacking; corallum fixed to fragment of substrate by small pedicel elliptical in cross section; pedicel not stereome reinforced. Pali and paliform lobes absent; lamellar columella prominent. Calicular edge smooth; septa often slightly exsert.
Occurrence
Late Eocene to middle Miocene: Victoria, South Australia, Tasmania.
Etymology
The Greek prefix peri, meaning near, prefaces Placotrochus to show its close relationship to that genus. Gender: masculine.
Remarks
Periplacotrochus differs from Flabellum in having a lamellar columella, and from Placotrochus by being pedicellate (not transversely dividing). Key to the species of Periplacotrochus is shown in Table 2.
Table 2 Key to the species of Periplacotrochus.
Periplacotrochus deltoideus (Duncan, Reference Duncan1864) new combination
Figure 2 (1–4) Periplacotrochus deltoideus, USNM 67953, Balcombe’s Bay, Victoria, middle Miocene, lateral, edge, calicular, and oblique calicular views, respectively. (5–7) P. corniculatus, holotype, NMV P27069, lateral, edge, and calicular views, respectively. (8–12) P. elongatus, USNM 10674, Muddy Creek, Victoria, middle Miocene: (8) specimen showing lamellar columella; (9–12) lateral, edge , calicular, and oblique calicular views of same specimen. (13–15) P. pueblensis, holotype, NMV P27106, lateral, edge, and calicular views, respectively. (1–6, 8–14) Scale bars=10 mm; (7, 15) scale bars=5 mm.
1864 Placotrochus deltoideus Reference DuncanDuncan, p. 164, 167, pl. 5, fig. 5a–c.
1870 Placotrochus deltoideus; Reference DuncanDuncan, p. 300, 309, 311, pl. 20, fig. 4a, b.
1875 Placotrochus deltoideus; Reference Tenison-WoodsTenison-Woods, p. 17.
1876 Placotrochus deltoideus; Reference DuncanDuncan, p. 341.
1878b Placotrochus deltoideus: Reference Tenison-WoodsTenison-Woods, p. 315.
1878c Placotrochus deltoideus; Reference Tenison-WoodsTenison-Woods, p. 65.
1927 Placotrochus deltoideus; Reference FelixFelix, p. 417.
1989b Placotrochus deltoideus; Reference CairnsCairns, p. 75, pl. 40, fig. a–c.
Undated Placotrochus deltoideus; Reference Fitzgerald and SchmidtFitzgerald and Schmidt, p. 3 (color illustration).
Types
Five syntypes are deposited at the NHMUK (PI R29269-29273). Type locality: Muddy Creek, Victoria (Balcombian, middle Miocene).
Diagnosis
Corallum with five cycles (96 septa) of hexamerally arranged septa.
Occurrence
Janjukian (=Rupelian, late Oligocene): Torquay, Victoria. Balcombian (=Langhian, middle Miocene): Muddy Creek, Balcombe’s Bay, and Beaumaris, Victoria. Miocene: northwestern Tasmania.
Description
The corallum has a bimodal edge angle, the lower thecal edges making an angle of 71º–87º, the upper thecal edges diverging at only 48º–60º (Fig. 2.1); the lower thecal edges are acute and usually bear a low, discontinuous crest (variety bursarius of Duncan, Reference Duncan1864). The thecal faces are planar (not convex), having a face angle of 30º–39º. The largest corallum (USNM 67953, Fig. 2.1–2.4) is 27.2 mm in height and 27.2 x 19.3 mm in calicular diameter. The calice is elliptical, the GCD:LCD ranging from 1.4 to 1.7; the H:GCD ranges from 1.0 to 1.1. The greater axis of the small (1.5–1.6 x 0.95–1.0 mm), elongate pedicel is aligned with the GCD. Each thecal face is ribbed with low C1–3 (Fig. 2.1).
Above a GCD of 20 mm the septa are consistently arranged in five complete cycles (Fig. 2.3–2.4): S1–3>S4>S5 (96 septa). Below that diameter, coralla have only 20–22 major septa (not 24) and a corresponding lesser number of higher cycle septa. The S1–3 have slightly sinuous axial edges, the lowermost edges fusing with the columella. The S4 are about half the width of the S1–3, and the S5 are rudimentary, but consistently present. The fossa is of moderate depth and contains a prominent lamellar columella, consisting of a single thick lamella.
Materials
Muddy Creek (Clifton Bank), Victoria, middle Miocene, one corallum, USNM 67955 (topotypic). USGS 10674, Muddy Creek, Victoria, middle Miocene, eight coralla, USNM M353589 (topotypic). Balcombe’s Bay, Victoria, Balcombian (middle Miocene), five coralla, USNM 67953 and USNM M353578. USGS 10809, Mornington, Balcombe’s Bay, Victoria, middle Miocene, eight coralla, USNM 77072 and USNM M353921. USGS 10808, Torquay, Victoria, Janjukian (late Oligocene), six coralla, USNM M353590.
Remarks
Periplacotrochus deltoideus is very similar to P. magnus, resembling a smaller version of that species. However, P. deltoideus can be distinguished by having a smaller corallum, straight (not convex) thecal faces and thus a less full corallum, consistently 96 hexamerally arranged septa (vs. three size classes of septa, the largest counting 28–32), consistently crested lower thecal edges, and ribbed C1–3.
Periplacotrochus corniculatus (Dennant, Reference Dennant1899) new combination
1899 Placotrochus corniculatus Reference DennantDennant, p. 115, pl. 2, fig. 3a–b.
1927 Placotrochus corniculatus; Reference FelixFelix, p. 416.
1981 Placotrochus corniculatus; Reference BellBell, p. 11 (type deposition).
1989b Placotrochus corniculatus; Reference CairnsCairns, p. 75 (mentioned).
Types
Holotype deposited at the NMV (P27069); paratype NMV P27070 (Blanche Point); Dennant (Reference Dennant1899) mentioned a third specimen, which would also be a paratype. Type locality: Adelaide bore, South Australia, Miocene according to Felix (Reference Felix1927).
Diagnosis
Corallum with four cycles (48 septa) of hexamerally arranged septa; corallum cornute.
Occurrence
Known only from the type locality.
Description
This species is known only from three specimens. The holotype is a cornute specimen (Fig. 2.5), curved about 75º. It is 20 mm in height and 9.0 x 6.5 mm in calicular diameter, having a H:GCD of 2.2. The GCD:LCD range of the type series is 1.21–1.56. The edge angle is about 13º; the face angle could not be computed from the figures or text. The septa are arranged in four complete cycles: S1–2>S3>S4 (48 septa). The S1–2 are slightly exsert with strongly sinuous axial edges. The S3 are almost as wide as the S1–2, and the S4 are relatively small. The fossa contains a typical lamellar columella.
Remarks
This species is distinguished from the others in the genus by having a regularly curved (cornute) corallum (see Key in Table 2).
Periplacotrochus elongatus (Duncan, Reference Duncan1864) new combination
1864 Placotrochus elongatus Reference DuncanDuncan, p. 163, 167, pl. 5, fig. 4a–e.
1870 Placotrochus elongatus; Reference DuncanDuncan, p. 300, 311, 312, 313, pl. 20, fig. 3a–c.
1878b Placotrochus elongatus; Reference Tenison-WoodsTenison-Woods, p. 314.
1927 Placotrochus elongatus; Reference FelixFelix, p. 417.
1989b Placotrochus elongatus; Reference CairnsCairns, p. 75.
Types
The holotype is deposited at the NHMUK (PI R29274). Type locality: Muddy Creek, Victoria (middle Miocene).
Diagnosis
Corallum with four cycles (48 septa) of hexamerally arranged septa; corallum straight to irregularly bent (but not cornute); calice elliptical in cross section.
Occurrence
Janjukian (late Oligocene): Torquay, Victoria. Balcombian (middle Miocene). Muddy Creek, Aire River, Grice’s Creek, Victoria. Miocene: Table Cape, Tasmania, Mount Gambier, Cape Otway, Victoria.
Description
The corallum is shaped as an isosceles triangle (Fig. 2.9), the edge angle ranging from 17º–29º and the thecal edges being straight. The thecal faces are planar (not convex), having a face angle of 12º–22º (Fig. 2.10). The lower thecal edges are acute but not crested; the upper thecal edges are evenly rounded, producing an elliptical calice having a GCD:LCD of 1.50–1.67. The H:GCD ranges from 1.6 to 3.2, thus the name elongatus. One of the larger coralla (USNM 77069) measures 11.3 x 7.3 mm in calicular diameter and 22.3 mm in height. The pedicel is small, about 1.9 x 1.0 mm in diameter, the longer axis aligned with the GCD.
The septa are hexamerally arranged in four complete cycles: S1–2>S3>S4 (48 septa), even in coralla as small as 6.0 mm GCD. S1–2 are exsert, their axial edges quite sinuous, and their lower axial edges fused to the lamellar columella. The S3 are about four-fifths the width of the S1–2, much less sinuous, and they fuse with the columella. The S4 are two-thirds the width of the S3, their axial edges not attaining the columella. The fossa is deep and contains a prominent sharp lamellar columella, characteristic of the genus (Fig. 2.11, 2.12).
Materials
USGS 10674, Muddy Creek, Victoria, Balcombian (middle Miocene), 39 coralla, USNM 77069 (topotypic). Clifton Bank, Muddy Creek, Victoria, Balcombian, eight coralla, USNM 67956 (topotypic). Grice’s Creek (near Frankston), Victoria, Balcombian, six coralla, USNM M353579. Sunnyside Beach (Grice’s Creek), Mornington, Victoria, Balcombian, 1 corallum, USNM 1283880. USGS 10808, Torquay, Janjuk Creek, Janjukian (late Oligocene), one corallum, USNM 1283878. Bird Rock Cliff, Torquay, Janjukian, two coralla, USNM 1283879.
Remarks
Periplacotrochus elongatus is easily distinguished from other species by its elongate slender corallum (GCD:H=1.6–3.2) (see also the Key in Table 2).
Periplacotrochus pueblensis (Dennant, Reference Dennant1903) new combination
1903 Placotrochus pueblensis Reference DennantDennant, p. 212, pl. 2, fig. 1a–b.
1981 Placotrochus pueblensis; Reference BellBell, p. 11 (type deposition).
1989b Placotrochus pueblensis; Reference CairnsCairns, p. 75 (mentioned).
Types
The holotype is deposited at the NMV (P27106). Type locality: Spring Creek (The Ledge), Victoria, Janjukian (late Oligocene).
Diagnosis
Corallum with four cycles (48 septa) of hexamerally arranged septa; corallum straight to irregularly bent (but not cornute); calice round in cross section.
Occurrence
Known only from the type locality.
Description
The species is known from only one specimen, the holotype. It is essentially conical (Fig. 2.13, 2.14): 17.8 mm tall (as measured from the figure) and 10.4 x 10.2 mm (as measured from the figure) in calicular diameter (GCD:LCD=1.02, essentially circular). The angle of thecal divergence is uniformly about 27º. The epitheca is porcellaneous. The septa are arranged in four complete cycles: S1–2>S3>S4 (48 septa). The S1–2 are slightly exsert, whereas the S3 are about three-fourths the width of the S1–2, and the S4 are about half the width of the S3. The fossa is deep, containing a somewhat unusual (?aberrant) double-lamellar columella (Fig. 2.15).
Remarks
Periplacotrochus pueblensis is distinctive in having a circular calice and 48 septa, although it is very similar to P. inflectus, as described in the account of that species (see also Key to species in Table 2).
Periplacotrochus inflectus (Dennant, Reference Dennant1903) new combination
Figure 3 (1–4) Periplacotrochus inflectus: (1, 2, 4) NMV P27107, syntype, lateral, edge, and calicular views, respectively; (3) calicular view of specimen NMV P134078. (5–8) P. cudmorei, holotype, USNM 1283876, lateral, edge, calicular, and oblique calicular views, respectively. (9–16) P. magnus: (9–11) holotype, NMV P27114, lateral, edge, and calicular views, respectively; (12) USNM 67954, damaged specimen showing its lamellar columella; (13–16) USNM 67954, lateral, edge, calicular, and oblique calicular views of same specimen, respectively. (1, 2, 5–16) Scale bars=10 mm; (3, 4) scale bars=5 mm.
1903 Placotrochus inflectus Reference DennantDennant, p. 213, pl. 2, fig. 2a–c.
1981 Placotrochus inflectus; Reference BellBell, p. 11 (type deposition).
1989b Placotrochus inflatus (sic); Reference CairnsCairns, p. 75.
Types
Four syntypes are deposited at the NMV (P27107 and 27108), see Bell (Reference Bell1981). Two additional specimens (?syntypes) are also deposited at that museum: P134078 and P146120. Type locality: “Eocene clays at Brown’s Creek, Aire R. district” (Dennant, Reference Dennant1903, p. 213), Victoria, middle to late Eocene.
Diagnosis
Corallum with four cycles of hexamerally arranged septa but lacking eight S4, resulting in 40 septa; corallum straight to irregularly bent (but not cornute); calice round in cross section.
Occurrence
Known only from the type locality.
Description
The species is known from only four specimens, two of them poorly preserved. The specimen described and illustrated by Dennant (Reference Dennant1903) is ceratoid (almost conical, Fig. 3.1, 3.2), 7.5 x 6.5 mm in calicular diameter (GCD:LCD=1.15), and 16 mm in height (H:GCD=2.13). The edge angle is about 23º. The epitheca is porcellaneous and the C1–2 are slightly raised. The pedicel attachment is elongate, as is typical for the genus. The septa are arranged in four cycles (S1–2>S3>S4), but the fourth cycle is incomplete, lacking four pairs of S4 in the lateral half systems, thus resulting in only 40 septa. The S1–2 are thick, their axial edges fused to the columella. The S3 and S4 are much thinner, the S3 about three-fourths the width of the S1–2, and the S4 small. The lamellar columella is prominent (Fig. 3.3, 3.4).
Remarks
Periplacotrochus inflectus is very similar to P. pueblensis and may simply represent an earlier ontogenetic stage in which the corallum is slightly smaller and thus has an incompletely developed fourth cycle of septa. Most other characters are similar or the same. However, the stratigraphic occurrence of P. inflectus is much earlier (middle to late Eocene) than that of P. pueblensis (late Oligocene). Clearly, more specimens are needed to resolve this distinction.
Periplacotrochus cudmorei new species
Types
Holotype: Clifton Bank, Muddy Creek, Victoria, Balcombian (middle Miocene), USNM 1283876. Paratypes: same locality as holotype, two, USNM 1283875. Bird Rock Cliffs, Torquay, Victoria, Janjukian (late Oligocene), four, USNM M353583. Spring Creek, Torquay, Victoria, Janjukian (late Oligocene), three, USNM 1283874. Type locality: Clifton Bank, Muddy Creek, Victoria, Balcombian (middle Miocene).
Diagnosis
Septa arranged in three size classes, having 20 primary septa and thus 80 septa.
Occurrence
Balcombian (middle Miocene): Muddy Creek, Victoria. Janjukian (late Oligocene): near Torquay, Victoria.
Description
The corallum is triangular in lateral view (Fig. 3.5), the thecal edges forming an angle of 38º–62º and the planar faces forming a lesser angle of 30º–38º (Fig. 3.6). The lower thecal edges are acute and bear low crests, whereas the upper thecal edges are rounded. The holotype measures 18.2 x 11.7 mm in calicular diameter and is 20.0 mm in height. The GCD:LCD ranges from 1.42 to 1.68, and the H:GCD ranges from 0.8 to 1.1 but is usually over 1.0. The greater axis of the elongate (1.6–1.9 x 0.7–1.1 mm in diameter) pedicel attachment is aligned with the GCD. The 20 costae on the thecal faces are sometimes slightly elevated.
The septa are consistently arranged in three size classes: 20:20:40, resulting in 80 septa. The primaries are exsert as much as 1.5 mm and have moderately sinuous axial edges, their lower axial edges fusing to the lamellar columella. The secondary septa are about two-thirds the width of the primaries, have less sinuous axial edges, and do not reach the columella. The tertiaries are rather small. The fossa is shallow and contains a prominent lamellar columella (Fig. 3.7, 3.8).
Etymology
Named in honor of Francis Alexander Cudmore (1892–1956), enthusiastic marine paleontologist who collected in Victoria and Tasmania and provided most of the Australian Tertiary corals donated to the Smithsonian in 1923.
Remarks
This species is unique in having decameral septal symmetry (see Key in Table 2).
Periplacotrochus magnus (Dennant, Reference Dennant1904) new combination
1904 Placotrochus magnus Reference DennantDennant, p. 54, pl. 22, fig. 4a–c.
1981 Placotrochus magnus; Reference BellBell, p. 11 (type deposition).
1989b Placotrochus magnus; Reference CairnsCairns, p. 75 (mentioned).
Types
The holotype (P27114) and paratype (P27115) are deposited at the NMV. Type locality: Spring Creek (16 miles south of Geelong), Victoria, Australia, Janjukian (late Oligocene).
Diagnosis
Septa arranged in three size classes, but with 27–32 primary septa, resulting in 104–128 septa.
Occurrence
Janjukian (late Oligocene): Torquay and Spring Creek (near Geelong), Victoria, Australia.
Description
The corallum has a bimodal edge angle, the lower thecal edges making an angle of 91º–98º (Fig. 3.9, 3.13), whereas the upper thecal edges diverge at only 34º–46º; the lower thecal edges are acute and bear a low crest, whereas the upper thecal edges are less acute and rounded near the calice. The thecal faces are planar to slightly convex, making a face angle of 36º–45º (Fig. 3.10, 3.14). The largest corallum (the holotype) is 37 x 26 mm in calicular diameter and 33 mm in height. The calice is elliptical, the GCD:LCD ranging from 1.36 to 1.66; the H:GCD ranges from 0.89 to 1.21. The pedicel is small (1.8–2.0 x 0.75–1.05 mm), the greater axis aligned with the GCD.
The septa are arranged in three size classes, the holotype having septa arranged as 28:28:56 (112 septa), but other coralla have 27–32 primaries and a total of 104–128 septa. The primary septa are slightly exsert and have slightly sinuous axial edges, the lowermost edges fusing to the lamellar columella. The secondaries are about half the width of the primaries and do not attain the columella. The tertiaries are small to rudimentary but consistently present over a GCD of about 30 mm. The fossa is of moderate depth and contains a prominent lamellar columella, usually consisting of a single, thick (0.5 mm) lamella (Fig. 3.12), but sometimes as two or three aligned lamellae (Fig. 3.15, 3.16).
Materials
Torquay (near Geelong), Victoria, Janjukian (late Oligocene), two coralla, USNM 67954. Spring Creek (also known as Bird Rock Cliffs), Janjukian (late Oligocene), four coralla, USNM 67952.
Remarks
Periplacotrochus magnus is compared to the closely related P. deltoideus in the account of the latter species.
Periplacotrochus elegans Tenison-Woods, Reference Tenison-Woods1878a, nomen dubium
1878a Placotrochus elegans Reference Tenison-WoodsTenison-Woods, p. 189, pl. 1, fig. 1, 1a.
1927 Placotrochus elegans; Reference FelixFelix, p. 417.
1971 Placotrochus elegans; Reference FletcherFletcher, p. 32.
1989b Placotrochus elegans; Reference CairnsCairns, p. 75.
Types
Holotype deposited at the Australian Museum (F.1697) (Fletcher, Reference Fletcher1971). Type locality: Muddy Creek, Victoria, Balcombian (middle Miocene).
Diagnosis
The species cannot be diagnosed as it is probably a juvenile stage of another species.
Occurrence
Known only from the type locality.
Description
The holotype is only 3 mm tall, with a compressed calice (3.0 x 1.5 mm in diameter, GCD:LCD=2; H:GCD=1) and an elongate basal attachment (1.65 mm in greater diameter). The edge angle is about 21º. According to Tenison-Woods’ (Reference Tenison-Woods1878a) illustration, there are 16 larger septa and 16 smaller septa and a prominent lamellar columella.
Remarks
The species is known from only one small specimen, undoubtedly the juvenile stage of a larger specimen, which shows the characteristic basal attachment of a Periplacotrochus. Three species of Periplacotrochus are known from the fossil beds of Muddy Creek, Victoria (P. elongatus, P. deltoideus, and P. cudmorei), and this juvenile specimen could be any of these three. A specimen of similar size and shape is shown herein as Figure 4.1 and 4.2. It is therefore suggested to treat the name as a nomen dubium, as defined by the International Code of Zoological Nomenclature (article 75.5).
Figure 4 Juvenile specimen of Periplacotrochus, similar to that described as P. elegans, USNM 67999, Muddy Creek, Victoria, middle Miocene: (1) lateral view of specimen showing characteristic broad attachment to a shell fragment; (2) stereo view of calice showing 32 septa. Scale bars=1 mm.
Acknowledgments
I thank F. Holmes and R. Schmidt (National Museum of Victoria, Melbourne) for permission to study the Dennant types deposited at that museum and for providing images of those specimens. I also thank R. H. Ford for photographing the NMNH specimens and composing them into Figures 1–4.
