LAMELLIBRANCHIA TA. 
397 
The identification with Mr. Conrad’s species was made from a lithographed 
figure which has never been published. 
Formation and locality. In the shales of the Hamilton group, in Schoharie 
county, N. Y. 
Sphenotus subtortuosus. 
PLATE LXV, FIGS. 18, ]9. 
Sanguinolites subtortuosus, Hall. Prelim. Notice Lamellibranchiata, 2, p. 41. 1870. 
“ “ “ Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 65, figs. 18, 19. 
18S3. 
Shell large, robust, trapezoidal; length more than twice the height; basal 
margin constricted anterior to the middle and gently curving to either 
extremity. Posterior extremity doubly truncate. Cardinal margin straight 
or gently arcuate, and very slightly declining posteriorly. Anterior end 
short, abruptly declining from the beak and narrowly rounded below. 
Valves moderately convex posteriorly, becoming gibbous in the middle 
and umbonal region. 
Beaks sub-anterior, attenuate, appressed and incurved, rising but little 
above the hinge-line. Umbonal slope marked by a low ridge, extending to 
the post-inferior extremity, becoming undefined below the middle of its 
length. Post-cardinal slope convex, marked along the middle of its width 
by a low ridge, extending from the beaks to the posterior extremity, produc¬ 
ing an angularity in the margin. Cincture broad, distinct, extending from 
the beaks and constricting the basal margin a little anterior to the middle. 
Surface marked by fine concentric striae, which in some parts of the shell 
are fasciculate, producing low undulations of the surface ; and also by fine 
radiating striae, which are often not well-preserved. 
Ligamental area narrow ; characters of the interior unknown. 
Two specimens measure respectively 50 and 55 mm. in length, and 23 and 
26 mm. in height. 
This shell resembles the preceding, but is a larger and more robust form, the 
valves are more gibbous; the umbonal ridge is distinctly arcuate, not so 
51 
