. LAMELLIBRANCHIATA. 251 



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 siriooth, but show- 

 ing lamellose stri« on the pallial mai-gin. 



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 count}', N. Y. 



Mytilarca umbonata. 



PLATE XXXII, FIGS. 1-7. 

 Mj/tllarca umbo7iata,'ii>i.LL. Pul. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 32, tig-s. 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 sloj)e 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 mm., l)readtli 19 mm. 



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

 form ; the beaks more elevated and erect. 



Formation and localities. In the Middle Chemung group, Ithaca, N. Y., and 

 near Mansfield, Pa. 



33 



