454 PALEONTOLOGY OF NEW YORK. 



above. In its usual condition of preservation the shell is very much 

 depressed. 



. Boaks at about the anterior third, prominent. Umbonal slope sub-angular, 

 defined above, less prominent below. 



Surface marked by fine concentric striae of growth, which are partially 

 preserved in the cast. 



Three specimens measure respectively 33, 35 and 43 mm. in length, and 

 25, 29 and 30 mm. in height. 



This species resembles S. appressus in form and proportions and is probably 

 only a variety of that species which lived under different conditions. 



Formation and localities. In the Chemung group, at Chemung Narrows ; near 

 Ithaca and Cortland, and in Lindley township, Steuben county, N. Y. ; also in 

 Tioga and Susquehanna counties, Pa. 



ScHizoDus Chemungensis, var. quadrangularis. 



PLATK LXXV, HGS. 81-34, 36. 



In part Scltizodux qitadrangularU, Hall. Prelim. Notice Lamellibranchiata, 2, p. 96. 1870. 



" Cytherodim {Sclmodits) qtuidrangulari.1. Hall. Pal. N. Y., vol. v, i)t. 1. Plates and Explanations : 



PI. 75, figs. 31-36. 1883. 



Shell large, rhomboidal ; length somewhat greater than the height ; basal 

 margin broadly rounded. Posterior extremity pointed below and truncate 

 above. Cardinal line about half the length of the shell. Anterior end large, 

 broadly rounded. 



Valves depressed-convex below, becoming convex or gibbous in the mid- 

 dle and upper portions. 



Beaks between the middle and anterior third, prominent, rising above the 

 hinge-line. Umbonal slope distinctly angular, defined, extending to the 

 post-inferior extremity. 



Surface marked by fine concentric striae, which are partially preserved in 

 the casts. The umbonal slope, in the cast, is marked by a sharp angularity 

 extending from the beak half way to the post-inferior extremity. The car- 

 dinal slope is marked by a furrow along the middle for half its length. 



The pallial line extends at some distance from the margin, terminating in 



