LAMELLIBRANCHIATA. 455 



an elongate posterior muscular impression. Anterior muscular impression 

 strongly marked. 



Three specimens measure respectively 30, 34 and 38.5 mm. in length, and 

 26, 28 and 33 mm. in height. 



This form appears to be only a variety of S. Chemungensis, with a more erect, 

 distinctly quadrangular outline, and having a wider post-cardinal slope. 



Formation and localities. In the Chemung group, on Cayuta creek and at 

 Factory ville in Tioga county ; on the banks of Chemung creek and at Ithaca, 



N. Y. 



SCHIZODUS OBLATUS. 



PLATE LXXV, FIGS. 43, 44. 



Schizodus oblatus, Hall. Prelim. Notice Lamellibranchiata, 2, p. 97 1870. 



In part Oytherodcm (SchUodiis) oblatits. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 75, 



fig-8. 41-45. 1883. 



Shell of medium size or larger, broadly sub-elliptical; length one-fourth 

 greater than the height; basal margin regularly curving. Post-inferior 

 extremity obtusely angular and the margin above sub-truncate. Cardinal 

 line less than half the length of the shell. Anterior end broadly rounded. . 



Valves convex in the lower portion, becoming gibbous or ventricose in the 

 middle and above, depressed-convex along the middle of the lower portion of 

 the valve. 



Beaks a little anterior to the middle, prominent, broad and rounded. 

 Umbonal slope obtusely sub-angular, extending to the post-inferior extrem- 



ity. 



Surface marked by fine concentric striae, which are but ftiintly preserved 

 in the cast. 



Muscular impressions strongly marked. The posterior scar, on the cardi- 

 nal slope, is at about the middle of the height of the valve. 



Two specimens measure respectively 30 and 32 mm. in length, and 22 and 

 24.5 mm. in height. 



This form differs from any other of the genus, here noticed, in the prominent 



