STROPHODONTiE OF THE HAMILTON GROUP. 
99 
Ventral valve very little convex, the greatest convexity above the 
middle of its length, with frequently a few obsfeure concentric wrinkles 
near the apex, and sometimes upon the body of the shell : apex 
scarcely rising above the hinge-line, and slightly incurved. Dorsal 
valve gently concave, and often nearly flat. 
Area of the ventral valve usually less than a line in width, inclined at 
an angle of 40° to 50° to the plane of the margins and curved in the 
upper part, vertically striated in its whole extent and crenulate on 
the inner margin; sometimes a flat triangular space in the place of a 
foramen, with a narrow callosity in the middle, but this feature is not 
always observable. Area of the dorsal valve about half as wide as that 
of the ventral, gently curved outwards, leaving an angle between the 
two of more than 90°: the centre is marked by a narrow callosity or 
an impressed space. 
Surface covered by fine subequal striae, those of the ventral valves being 
the finer, extremely sharp and often gently undulating, increasing 
both by bifurcation and intercalation, and crossed by fine, even 
concentric striae. In some specimens the longitudinal striae rise at fre¬ 
quent and regular intervals into minute granules, evidently the bases 
of minute spines, which have covered the surface of the ventral valve. 
The dorsal valve is marked by fine even rounded striae which are can¬ 
cellated by close concentric striae, and the same obscure concentric 
undulations as are observed on the surface of the ventral valve. Very 
rarely there is some interruption to the regularity of the striae, appa¬ 
rently owing to an injury which has often caused the concentric striae 
to curve towards that point, and the radiating striae to converge, ma¬ 
king a kind of seam or cicatrix. 
In specimens of the same species from Illinois and Iowa, the striae on 
the ventral valve are less sharp, and arranged in fascicles of four to six 
finer ones between the stronger and more elevated striae. In the speci¬ 
mens from the Chemung group, the striae are often very irregular, rising 
at intervals into elongate pustulose elevations, and again subsiding to 
delicate lines. 
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