SYNOPSIS OF GENERA. 
xlvii 
interrupted at the outer margin of the lamellae. The valves are crossed 
obliquely by a shallow byssal sinus. 
Hinge-line marked by a long fold or lateral tooth in one valve. Cardinal 
teeth not determined. Ligament external, strong, leaving in the cast a deep, 
elongate groove. Anterior muscular impression small, situated just within the 
anterior margin of the shell. Posterior muscular impression large and strong, 
situated on the cardinal and umbonal slope a little behind the middle of the 
length. Pallial line parallel with the margin of the shell. 
The largest of our species is not known to reach a length of 30 mm., but 
Mr. Billings has described C. distinda from the Gaspe limestone series, which 
he says has a greatest length of seventeen lines. 
This genus includes a very natural and characteristic group of shells, 
which begin their existence in the Silurian rocks, and extend to the coal 
measures ; occurring in the Trenton, Niagara, Lower Helderberg, Upper Helder- 
berg, Hamilton, Chemung and Waverly groups. They are usually readily 
recognized by their form, the inequality of the valves and the lamellose surface. 
In the C. planulata , from the Schoharie grit, the umbonal slope is limited above 
by a distinct furrow, which is more pronounced on the cast. In addition to the 
ordinary concentric surface markings, some of the species show radiating 
striae originating at the apex of the valve, and these are sometimes crossed 
by others which are vertical to the direction of the lamellae. Of the species 
originally described under this generic term, in the third volume of the Paleon¬ 
tology of New Yoke, only C. lamellosa and C. crassa can be regarded as strictly 
belonging to the genus. 
This genus is well known in the Palaeozoic rocks of Europe, 
Examples: Cijpricardites planulata , pi. lxxix, figs. 1-5. 
Cypricardites indenta , pi. lxxix, figs. 6-16, 23. 
Palaeanatina, Hall (Prelim. Notice Lamellibranchiata, 2, p. 85. 1870). 
[Type, Palaeanatina typa, Hall.] 
Shell inequivalve ; left valve larger than the right, inequilateral, transversely 
elongate, narrowly sub-elliptical or sub-trapezoidal. Anterior end short, 
