STROPHODONTiE OF THE HAMILTON GROUP. 99 



Ventral valve very little convex, the greatest convexity above the 

 middle of its length, with frequently a few obscure 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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