Tertiary and Recent marine gastropods include in their ranks a complement of mechanically sturdy forms unknown in earlier epochs. Open coiling, planispiral coiling, and umbilici detract from shell sturdiness, and were commoner among Paleozoic and Early Mesozoic gastropods than among younger forms. Strong external sculpture, narrow elongate apertures, and apertural dentition promote resistance to crushing predation and are primarily associated with post-Jurassic mesogastropods, neogastropods, and neritaceans. The ability to remodel the interior of the shell, developed primarily in gastropods with a non-nacreous shell structure, has contributed greatly to the acquisition of these antipredatory features.

The substantial increase of snail-shell sturdiness beginning in the Early Cretaceous has accompanied, and was perhaps in response to, the evolution of powerful, relatively small, shell-destroying predators such as teleosts, stomatopods, and decapod crustaceans. A simultaneous intensification of grazing, also involving skeletal destruction, brought with it other fundamental changes in benthic community structure in the Late Mesozoic, including a trend toward infaunalization and the disappearance or environmental restriction of sessile animals which cannot reattach once they are dislodged. The rise and diversification of angiosperms and the animals dependent on them for food coincides with these and other Mesozoic events in the marine benthos and plankton.

The new predators and prey which evolved in conjunction with the Mesozoic reorganization persisted through episodes of extinction and biological crisis. Possibly, continental breakup and the wide extent of climatic belts during the Late Mesozoic contributed to the conditions favorable to the evolution of skeleton-destroying consumers. This tendency may have been exaggerated by an increase in shelled food supply resulting from the occupation of new adaptive zones by infaunal bivalves and by shell-inhabiting hermit crabs.

Marine communities have not remained in equilibrium over their entire geological history. Biotic revolutions made certain modes of life obsolete and resulted in other adaptive zones becoming newly occupied.

Poecilozonites (Gastrelasmus) is an important component of the endemic land snail fauna of Pleistocene Bermuda. The type species P. circumfirmatus Redfield usually occurs in sympatry with its congener P. discrepans Pfeiffer, though each species is found alone at several localities. The species are less alike morphologically where they occur together than where they are allopatric. This allopatric convergence and sympatric divergence strongly suggests the biological interaction known as character displacement, often documented for living populations. The relatively complete fossil record of Bermuda offers advantages for studying this phenomenon. Collections can be made from a variety of microhabitats occupied through time. Statistical analysis of 1,600 individuals collected from more than 100 localities indicates that interspecific variation is primarily a function of the presence or absence of a congener and depends to a lesser degree on microhabitat. P. circumfirmatus undergoes a smaller morphological change between allopatry and sympatry than does P. discrepans. Study of relative abundances suggests that P. discrepans may have been competitively inferior, though no functional bases for differences between the species are known.

(1) Hydrodynamic principles and experiments with empty shells predict that pressure distributions around brachiopod shells generated by ambient currents should, depending on the orientation of the shell relative to the current, either augment or oppose the ciliary-driven flow of water through the lophophore. For living articulate brachiopods with plectolophes or spirolophes, orientations where the anterior-posterior axis of the shell is parallel to the current direction should result in pressure distributions which oppose active pumping. This effect should be strongest when the excurrent region of the shell faces into the current. Orientations where the anterior-posterior axis is perpendicular to the current direction should result in pressure distributions which act in concert with active pumping, most strongly when one of the incurrent regions is directed into the current. These effects are independent of specific shell shape.

(2) Laqueus californianus and Terebratulina unguicula actively reorient to currents in the laboratory, preferring orientations where the anterior-posterior axis of the shell is perpendicular to the current and the right-left axis is parallel to the current. Both species may traverse an arc as great as 120° to achieve their final orientation. Hemithyris psittacea also will actively reorient to currents, moving towards orientations where the anterior-posterior axis is perpendicular to the current. The maximum rotation observed for H. psittacea was 45°. Terebratalia transversa never reoriented in the laboratory.

(3) Using epifaunal hydroid colonies as indicators of current direction, both Hemithyris psittacea and Terebratalia transversa are oriented in nature with the anterior-posterior axis of the shell perpendicular to the prevailing currents. While scuba diving, I confirmed this orientation phenomenon for T. transversa by direct measurement of the orientation of the brachiopods relative to prevailing currents.

(4) Larval Terebratalia transversa avoid areas with current speeds greater than about 0.2 cm/s during metamorphosis and show no orientation to the ambient currents immediately after metamorphosis. Post-metamorphic T. transversa can actively reorient on the pedicle. The orientation observed in adults is probably achieved by active reorientation to local currents of post-zygolophe juveniles.

(5) Threshold current speeds for reorientation in Laqueus californianus and Terebratulina unguicula are low and approximately equal to the excurrent pumping speeds of each species; dynamic pressure rather than viscous entrainment is probably the relevant factor determining reorientation behavior.

Psychological evidence suggests that the visual outline of an object is the most important character for discriminating differences in external morphology. External morphology is an important taxonomic character in describing living and fossil species, for example, the blastoid Pentremites. Comparison of different views of the same specimen utilizing Fourier series suggests that the skeleton of Pentremites commonly is not rotationally symmetrical and that the asymmetry is not associated with any specific ray. Analysis of a growth series indicates that the amplitudes of the second and third harmonics are significantly correlated with growth, which is demonstrated to be anisometric. Definable changes in the lateral outline can be attributed to changes in harmonic amplitudes and a wide range of morphological forms comparable to known taxa can be generated by systematically varying the amplitudes of the first four harmonics. The population used in this study probably represents Pentremites robustus Lyon and is from Bangor Limestone in the abandoned Moulton Quarry, Lawrence County, Alabama.

Models of “ideal” orthoconic shells having simple concave septa with minimal weight and maximal strength and analysis of 72 species of fossil orthocones and cyrtocones yield important insights into the physical principles underlying cephalopod shell design. The ideal septum is a spherical cap weighing only 77% of a hemispherical septum of equal strength. The septa of most longicones approximate this ideal shape while those of brevicones are less curved, probably owing to buoyancy problems. Increase in septal strength leads to weight increase unless the shell becomes more logiconic or septal spacing increases or both. However, increased spacing requires more cameral liquid for septum formation, thus reducing buoyancy. In ideal longicones, septal spacing resembles the cone radius for thick, strong septa but declines to half of the cone radius for thin, weak septa. In ideal intermediates and brevicones, spacings are respectively reduced by factors of about 2 and 4, with similar additional dependence on septal thickness. Most real septa resemble these ideal models.

The relative length of the body chamber to the phragmocone varies greatly between about 0.2 and 1.5, depending mainly on the wall thickness and to a lesser degree on the septal thickness, apical angle and body density. Removal of cameral liquid in the adult must be compensated for by additional growth to retain neutral buoyancy. The conditions for neutral equilibrium calculated for longicones with different “counterweights” indicate that: (1) cameral liquid only is least feasible; (2) half-and-half calcium carbonate and liquid results in one-third length and one-quarter volume reduction of the body chamber; (3) with calcium carbonate only, body chamber reduction is minimal. Real ‘counterweights’ appear to be intermediate between (2) and (3), providing the animal with horizontal stability, which is missing in (3). Most uncalcified siphuncles reduce the body chamber only slightly although they improve horizontal stability. If the wall attains full thickness only at the apical end of body chamber, the liquid-only ‘counterweight’ becomes feasible.