¹ The notation can be summarized briefly as follows: Ribs originating at the umbo are termed primaries and are numbered 1,2, 3, 4, etc., primary 1 being nearest the median line, and primary 4 furthest away. Secondary ribs which split off from the primaries are lettered a, b, c, etc., a being the earliest to split off, b the next, and so on. Tertiary ribs which split off from the secondaries are numbered 1, 2, 3, etc., 1 being the earliest to arise, and so on; and for ribs of higher orders the numerals continue to alternate with the letters. Thus, 2a1 denotes the earliest tertiary (1), on the earliest secondary (a), on the second primary (2).

Whether the rib splits off on the inside (i.e. towards the median line) or on the outside (i.e. away from the median line) of the parent rib is also very important, and Bancroft used the symbol ⁻ to indicate the inner ribs, and the symbol ° to denote the outer ribs. It must be emphasized that the terms “inner” and “outer” sides are always in reference to the immediate parent of the rib in question. Thus 2a°, which is the first secondary to arise on the outer side of primary 2, can bear a tertiary between it and the primary and which, lying on the inner side of its parent, is denoted 2a°1⁻.

¹ In Table VIII, columns 1 and 2 (Bancroft, 1945, p. 216), one would expect that since 1b¯ 1a° and 2ā1⁻ 2b⁻ in the respective ratios 93 and 85 (Resserella (s.l.) inconstans) to 55·7 and 21·3 (Resserella (s.I.) reushi) that the former has sharply raised “shoulders” on either side of a narrow mesial sinus and the latter a weakly convex dorsal valve. This is so, and it is the sharp accentuation of “shoulders” in Harknessella that contributes in the main to production of ribbing patterns distinct from Dalmanella and Resserella.

¹ If future work is carried out along these lines it would probably be expedient to accept a standard length of radius from the umbo (e.g. 1·5 cm.), at which to make notational readings. This will have the advantage of ensuring comparisons at approximately the same stage in the development of individuals. Compare, for instance, the considerable differences between readings at the shell periphery, and at a length of 1 cm. from the umbo, in the gerontic syntype of Dalmanella prototypa sp. nov. (pp. 168–9).

¹ Bancroft (1945, p. 189): “Wattsella (synonym of Dalmanella) in which the fulcral plates normally persist to maturity may be regarded as the parent genus of the others” (including Horderleyella).

² These two families are discussed here in the sense they are used by Bancroft and not as understood by Schuchert and Cooper. The former (1945, pp. 187–9) has advanced valid reasons for the removal of Horderleyella from this family, as reconstituted, to the Heterorthidae.