452 
PALAEONTOLOGY OF NEW YORK. 
SCHIZODUS GREGARIUS. 
PLATE LXXV, FIGS. 16-18 ; AND PLATE XCV, FIG. 27. 
Schizodus gregarius, Hall. Prelim. Notice Lamellibranchiata, 2, p. 97. 1870. 
Cyllurodon (Schizodus) gregarius, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 75, 
fig's. 16-18. 1883. 
Shell small, sub-rhomboid-ovate, sometimes sub-cuneate from compression; 
length from one-fourth to one-third greater than the height; basal margin 
regularly and broadly curving. Posterior extremity somewhat pointed 
below and obliquely sub-truncate above. Cardinal line short, less than half 
the length of the shell. Anterior end short, broadly rounded. 
Valves moderately convex below, becoming somewhat gibbous in the mid¬ 
dle and above. 
Beaks at about the anterior third, small, rising little above the hinge-line. 
Umbonal slope obtusely sub-angular, continuing to the post-inferior extremity. 
Surface marked by line concentric striae. 
Two specimens measure respectively 12 and 15 lira, in length, and 9 and 
12 mm. in height. 
This species is similar in form to young individuals of $. appressus, but is 
proportionally longer. It is also smaller and proportionally longer than speci¬ 
mens of S. rhombeus. 
Formation and locality. Abundant in some chocolate-colored, sandy shales of 
the Chemung group, on the banks of the Tioga river, near Covington, Pa. 
SCHIZODUS RHOMBEUS. 
PLATE LXXV, FIGS. 19-23. 
Cijpricardia? rhombea, Hall. Geol. Surv. N. Y., Rep. Fourth Dist., p. 291, t,. 139, figs. 2, 3. 1843. 
Schizodus rhombeus, Hall. Prelim. Notice Lamellibranchiata, 2, p. 97. 1870. 
Cytherodon ( Schizodus ) rhombeus. Hall. Pal N. Y., vol. v, pt, 1. Plates and Explanations: PI. 75, 
tigs. 19-23. 1883. 
Shell below the medium size, sub-rhomboidal; length a little greater than the 
height; margins regularly rounded from the anterior end to the post-inferior 
extremity. Posterior extremity truncate. Cardinal line short. 
