270 
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. 
Umbonal region a prominent sub-angular elevation, extending obliquely from 
the beak toward the post-basal margin, usually dying ont about the middle 
of the length of the shell. 
Test comparatively thick, strongly ornamented by regular concentric, 
rounded or sub-angular strke, 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. niytiloidcs, 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 State ; abundant 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. 
