L A MELLIBRANCHIATA. 257 



Hinge-line short. Beaks acute, extremely elevated, directed slightly 

 forward and curving over the cardinal line. 



The test has not been preserved. The casts are nearly smooth, but show- 

 ing lamellose striae on the pallia! margin. 



Interior unknown. 



A specimen of this species has a length of 36 mm., height 23 mm., breadth 

 22 mm. 



* 



This form is shorter, more gibbous, more attenuate towards the beak, and 

 less rounded on the posterior side than M. Chemungensis. 



Formation and locality. In the Schoharie grit, Schoharie county, N. Y. 



Mytilarca umbonata. 



PLATE XXXII, UGS. 1-7. 

 Mytilarca umbonata, Hail. Pal. X. V.. vol. v. pt. 1. Platesand Explanations : PI. 32, tigs. 1-7. Jan., 1883. 



Shell small to medium size: body ovate, acute above; length more than one- 

 third greater than the height ; ventral margin truncate, inflated at the byssal 

 opening; basal margin abruptly rounded. 



Valves equal, strongly convex in the lower part and narrowly gibbous in 

 the umbonal region. Ventral umbonal slope obtusely angular, concave 

 between this and the ventral margin. Dorsal side not alate. 



Hinge-line short. Beaks acute, elevated, nearly vertical, arching over the 



cardinal line. 



Test thin, marked by fine concentric lines of growth which become 



crowded and lamellose toward the pallial margin. 



Interior unknown. 



A specimen has a length of 33 mm., height 18 nun., breadth 19 mm. 



This species differs from M. Chemungensis in its shorter and more gibbous 

 form ; the beaks more elevated and erect. 



Formal/' lities. In the Middle Chemung group, Ithaca. N. V.. and 



near Mansfield, Pa. 



33 



