LAMELLIBRANCHIA TA. 
257 
Hinge-line short. Beaks acute, extremely elevated, directed slightly 
forward and curying over the cardinal line. 
The test has not been preserved. The casts are nearly smooth, but show¬ 
ing lamellose striae on the pallial 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, FIGS. 1-7. 
Mytilarca umbonata, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 32, figs. 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 mm., breadth 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 
