Cycloconcha milleri

• Ligament nymphs are absent
• Compressed, oval shell shape
• Smooth surface, very faint growth lines
• Simple pallial line

• It is also noteworthy that ligament nymphs are absent in many early heteroconchs. Walker (1998, p. 34) acknowledged their absence in Copidens and no evidence is observed for them in Actinodonta cuneata Philips, 1848 and Cycloconcha milleri (Meek, 1871). The latter species has a dorsally open, simple, narrow ligament fossette with, at most, only a very weakly developed lateral groove for the lamellar ligament.

Davis (1998):

• Pelecypod. Shell small, ovoid, compressed. The surface of the shell smooth, showing only faint growth lines. Waynesville.

Meek (1873)

• Shell ovate, rather compressed, or moderately convex, the greatest convexity being a little above and slightly in advance of the middle; extremities more or less narrowly rounded, basal margin longitudinally semielliptic in outline, the most prominent part being near the middle; cardinal margin sloping from the beaks at an angle of 130 to 135 degrees, and rounding into the lateral margins, the anterior slope being more abrupt than the other; beaks depressed, and somewhat obtuse, not very convex, placed more than one-third the length of the valves from the anterior end. Surface smooth, or only with obscure lines of growth.
• Length of a medium-sized adult specimen, 0.83 inch; height, 0.59 inch; convexity, 0.30 to 0.33 inch.
• In the shell here described, the hinge may be characterized as having one rather well-defined, sub-trigonal, or somewhat obliquely extended, cardinal tooth under the beak of the right valve, and a corresponding pit under the beak of the left valve, with sometimes an appearance of a small rudimentary tooth just in advance of this pit. Of posterior lateral teeth, there is, in the right valve, a long one ranging parallel to the cardinal margin, with a parallel furrow above and below it for the reception of two long posterior laterals in the left valve, the lower one of which is more prominent, and the upper merely linear or rudimentary. The furrow between these two posterior lateral teeth of the left valve is well defined and receives the corresponding tooth of the other valve. Below the lower of these furrows, on the posterior side of the right valve, there is a very slight marginal ridge, that possibly may sometimes assume the character of a second posterior lateral tooth; but it is most prominent anteriorly, where it connects with the cardinal tooth, of which is seems to be an oblique posterior prolongation, than a distinct tooth. On the anterior side, there is one short shorter, anterior lateral tooth in the right valve, also ranging parallel to the hinge margin, and above and below this a little furrow for the reception of two small anterior laterals in the left valve, which receive between them that of the right valve.
• The pallial line is certainly simple, and the muscular impressions well defined, the posterior one being larger than the other, and provided with a small accessory scar above, just under the posterior ends of the posterior lateral teeth. The ligament or cartilage was probably small and internal, as there are no traces of an external ligament to be seen, the valves fitting closely all along the hinge margin. No lunule or eschutcheon is to be seen in any of the specimens.

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