270 THE DEVONIAN FOSSILS OF CANADA WEST. 
Description.—Shell, varying greatly in size and shape, ovate or 
sub-triangular, very convex or irregularly sub-globular. In large 
specimens the ventral-valve is very convex, with an exceedingly promi- 
nent and tumid umbo; the outline on a side-view is strongly arched 
from the beak to the front, the most rapid curvature being in the 
upper half; the beak is incurved, but not in contact with the umbo 
of the dorsal valve; a broad shallow mesial sinus originates at the 
front margin and becomes narrower and shallower, until, at length, it 
dies out before reaching the beak ; im a front view the outline is sub- 
triangular. In small specimens the umbo and beak are proportionally 
much smaller, the form more nearly oval or nearly circular, and the 
mesial sinus occasionally obsolete. The dorsal valve is much the 
smaller, depressed convex, with a broad, slightly elevated mesial fold, 
on each side of which the shell is usually a little flattened, or even 
slightly concave ; the mesial fold sometimes not at all developed. 
The surface is covered with coarse, unequal, sub-angular, or obscurely 
rounded ribs, from one line to one line and a half in width. These 
ribs increase in number from the beak towards the front, both by 
sub-division and the insertion of smaller ones between the larger. On 
each side of the beak there is a smooth space. 
The only difference between this species and the well-known Pen- 
tamerus galeatus appears to be that, in the latter, the mesial sinus 
is‘on the dorsal and the fold upon the ventral valve. 
In the following figures a small] specimen is represented. 
Fig, 95. Fig. 96. 
Locality and Formation.—This species occurs in the Oriskany 
Sandstone, and Corniferous Limestone, in various places in the County 
of Haldimand, 
Collectors.—J. DeCew. E. DeCew. KE. Billings. 
