402 Thirty-fifth Report on the State Museum. 



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

 but showing lamellose striae on the pallial margin. 



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

 breadth 22 mm. 



This form is shorter, more gibbous, more attenuate toward the 

 beak, and less rounded on the posterior side than M. Chemung ensis. 



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

 N". Y. 



Mytilarca umbonata. 



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 be- 

 come crowded and lamellose toward the pallial margin. 



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. 



Mytilarca carinata. 



Mytilarca carinata, Hall. MS. 1877. 



Hall. S. A. Miller in Cat. Amer. Pal. Foss.,p. 197. 1877. 

 " " Hall. Pal. N. Y., vol v, pt. 1. Plates and Explanations : 



PI. 33, figs. 15-19. Jan., 1883. 



Shell of small or medium size, body obliquely sub-ovate ; length 

 nearly twice the height, except in young specimens; ventral 

 margin nearly straight or slightly concave for more than three- 

 quarters of the length of the shell, thence rounding abruptly into 

 the basal margin, then more gently rounded into the gently curving 

 dorsal margin. Valves equal, depressed-convex in the posterior part, 



