OF DENISON UNIVERSITY. 
41 
American species are S. cimeatus, wheeleri, ampins, perelegans, oc- 
cidentalis, and curtus. 
1. Schizodus affijiis, sp if) n, 
(Plate IV, Figs. 22 and 22a.) 
This species is represented by nearly perfect casts of both valves, 
which are similar in form though various in size. The description will 
refer to the specimens rather than the species they represent. Right 
valve ovate, sub-trigonal, greatest convexity being about one-third from 
beak and one-third in length ; anterior margin broadly curved, its 
curve varying little from an arc of a circle, a little truncated above ; 
lower margin much less curved to the sharp angle at its posterior ex- 
tremity, which is produced as the termination of a prominence begin- 
ning at the umbo as a sharp ridge extending as a low fold to the mar- 
gin ; posterior margin slightly convex, inclined forward entering the 
upper margin by a gentle curve. The beak is distant one-third the 
greatest length from the anterior margin and is but moderately convex. 
Surface apparently with fine concentric lines and deeper lines of 
growth. Length, 1.20, width, 1.03; convexity apparently not 
over 0.50. 
Schizodus cimeatus. Meek, differs most notably in the position of 
the beaks, which are located only one-quarter from the anterior mar- 
gin, and as a result, the posterior part of the hinge line is much longer 
and the anterior broad margin as seen in our shell is absent. Our speci- 
men as figured is possibly somewhat flattened. 
2. Schizodus sub-circularis, sp. n. 
( Plate IV, Fig. 24. ) , 
A still smaller species or variety from the same rocks differs chief- 
ly in having the beak near the middle line of the shell. It is known 
from a single left valve. The anterior and lower margins are contin- 
uous parts of almost a circle, which is only a little produced posterior- 
ly; the posterior margin is inclined; beak sub-median, posterior um- 
bonal ridge not sharp. The shell is moderately convex and is nearly 
smooth, except for the distant lines of growth. Length, .63, width, 
.59, convexity, about .20 (single valve). 
