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 York, 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 : Cypricardites planulata, pi. Ixxix, figs. 1-5. 



Cypricardites indenta, pi. Ixxix, 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, 



