276 PALEONTOLOGY OF NEW YORK. 



Hinge-line extending half, or sometimes more than half, the length of the 

 shell. 



Beaks sub-anterior, small, sharply angular, appressed, directed forward. 

 Uml)onal region a prominent sub-angular elevation, extending obliquely from 

 the beak toward the post-basal margin, usually dying out about the middle 

 of the length of the shell. 



Test comparatively thick, strongly ornamented by regular concentric, 

 rounded or sub-angular striae, which become lamellose and coalescing on the 

 anterior end of the valves, where they are less prominent. 



Anterior muscular impression strong, striated, situated just within the 

 anterior margin, with a small retractor scar above it. Posterior impres- 

 sion large and shallow. Pallial line moderately impressed. Hinge furnished 

 with a strong cardinal tooth just posterior to the beak in the left valve, and 

 a corresponding depression in the right valve. No proper lateral teeth have 

 been observed, but the cardinal margin is thickened and grooved from the 

 beak backward about half the length of the cardinal line. 



Several specimens showing the ordinary range of form have the respective 

 lengths of 50, 40, 40, 30, 52, 63, 68 mm., and heights of 27, 25, 30, 22, 32, 

 38, 38 mm. 



This species resembles M. mytiloides, but may be distinguished by its usually 

 smaller size in full-grown specimens, more prominent umbonal region, more 

 arcuate form, less prominent anterior end, and by the strong, regular, concentric 

 striae. It is a more elongate form than M. alta. 



Formation and localities. In the shales of the Hamilton group from the eastern 

 portion of the St^ate ; al)undant in the central portion of the State, and less 

 abundant in the softer calcareous shales in the western part of the State of 

 New York ; in the siliceous layers above the hydraulic limestone at the Falls 

 of the Ohio, at Charleston, Ind., and in the shales of this group, near Cumber- 

 land, Md. 



