OF DENISON UNIVERSITY. 83 
Dorsal valve very convex, the greatest convexity being near the 
beak ; beak much elevated, incurved. 
Ventral valve flattened, with a shallow mesial sinus, more marked 
along the posterior half of the shell, towards the beak ; beak but 
slightly elevated. 
Surface marked by twenty to twenty-four simple, strong, low, 
rounded, almost straight plications, equal to the broad, flattish spaces 
between them in width. 
From the form as described above there are many variations. 
The middle plication of the dorsal valve frequently manifests a tenden- 
cy towards becoming more elevated and almost forming a low carina 
towards the beak ; the other plications becoming more indistinct as they 
approach this point. They may also become more angular, more 
numerous, approaching thirty in number, and the spaces between them 
may become even narrower than the plications. Again, in a few 
specimens they divide dichotomously towards their extremities. The 
shells also vary considerably in size, from 1 8 to 43 mm. in breadth. 
Faint concentric striae may also appear in the depression between the 
plications ; lines of growth are still more rare. The anterior and lat- 
eral margins of the shell are rarely well defined in our specimens. 
Length, 18 mm.; breadth, 26 mm.; convexity, as nearly as can be 
determined by comparison of the separated valves, 8 to 9 mm., being 
accounted for mainly by the great convexity of the dorsal valve. 
The shell has been found only in the form of separated valves, the 
exterior surface exposed, the cardinal area invisible ; however, the few 
simple plications usually equaling the flat depressions between suffi- 
ciently determine this species. 
Locality and positio 7 t. Soldiers’ Home Quarries, Clinton Group ; 
very common in places. 
VII. Orthis hybrid a, Sowerby. 
{Plate XIII, Figs. 10 a, b.) 
Shell lenticular, greatest diameter at one third the length of the 
shell from the beak, wider than long ; valves nearly equal, hinge line 
about half the width of the shell. 
Dorsal valve convex, evenly rounded, beak less elevated than that 
of the ventral valve, cardinal area smaller, extending the length of the 
hinge line. 
