PALAEONTOLOGY OF NEW-YORK. 
30 
CRANIzE OF THE CHEMUNG GROUP. 
Crania koiii. 
PLATE III. 
Crania leoni : Hall, Thirteenth Report on the State Cabinet, 1860, p. 78. 
Shell subcircular, transverse or slightly elongate. Dorsal valve convex. 
Ventral valve concave, variable in form, more abruptly recurved to¬ 
wards the margin. Hinge-line straight, equal to a little- more than one- 
third the greatest width of the shell. Muscular impressions of the 
posterior adductors in the dorsal valve near the cardinal angles; the 
anterior ones near together and a little behind the centre, with two 
minute impressions a little anterior to the centre, marking the place 
of the retractor muscles. Ventral valve with.the posterior adductors 
corresponding to those of the dorsal valve; the anterior adductors 
occupying a subcircular area, and barely separated by an elevation 
marking the place of the protractor muscle. 
This species is known only, in the condition of casts of the interior. The 
dorsal side is moderately convex ; the apex apparently a little excen- 
tric on the posterior side. 
A single cast of a dorsal valve among the specimens is more regularly 
elliptical, a little more convex, and without the straight hinge-line : 
this one may perhaps be a distinct species. 
There are, likewise, in the rocks of the Chemung group, some other specimens 
of the character of Crania ; but, unfortunately, I have at this time no specimens 
in a condition to admit of their being satisfactorily described. 
Geological formation and locality. In the Chemung group : Leon, Cattaraugus 
county, N.Y. 
