80 PALEONTOLOGY OF OHIO. 



of the body of the shell to the direction of the hinge-line. Surface of the 

 shell ventricose, and often sub-carinate on the umbones and toward the 

 beaks, gradually and somewhat regularly sloping to the basal margin, 

 becoming attenuate and compressed toward the postero-cardinal region, 

 and abruptly truncate and even impressed on the anterior side. Beaks 

 acutely pointed, strongly incurved, terminal, and projecting above the 

 line of the hinge ; posterior end at right angles to the hinge straight or 

 rounded, or sometimes sloping obliquely backwards to the postero-basai 

 margin ; base sharply rounded. Anterior border of the valves excavated 

 below the beaks, forming a rather large byssal opening, which is usually 

 about half as wide as long when the valves are united. 



Surface of the valves marked by strong, radiating ribs, which are sim- 

 ple throughout, strongest on the body of the shell, and becoming finer on 

 the postero-cardinal region. On the uj)per portion of the shell the ribs 

 are flattened on the top, and often grooved in the center, giving them a 

 strongly duplicate character, but becoming smooth beloAv, the spaces be- 

 tween as narrow, or much narrower, than the width of the rib. The 

 ribs are crossed by fine, concentric, imbricating lines of growth, which 

 undulate as they cross the elevation. 



There seems to be considerable variation of form and surface charac- 

 ters among the specimens which appear to be j^roperly referable to this 

 species, so much so, that it is difhcult to give the featujes which appear 

 on difierent specimens in a single specific description, without appearing 

 to go beyond the limits of reasonable specific distinction. The general 

 form of the shell, in the first place, is so variable that one might select 

 individuals, which, if taken alone, might serve as types of several dis- 

 tinct species ; but in a collection of twenty or thirty specimens, taken 

 promiscuously, will be found examples that will connect the whole into 

 a series uniting the extremes of form. Among the specimens studied, 

 we find examples where the hinge-line is at right angles to the posterior 

 and anterior borders, these being parallel; in others the direction of the 

 posterior margin will vary ten, fifteen, or even a greater number of de- 

 grees from a right angle with the hinge-line, being directed obliquely 

 backwards; others, again, have the posterior and basal margins forming 

 an almost regular curve from the posterior extremity of the hinge-line 

 to the antero-basal portion of the shell. The base is sometimes broadly 

 rounded, and, again, obliquely and acutely rounded, with the longest 

 point much posterior to the middle of the shell. In the surface charac- 

 ters they also vary much in the form and number of the ribs, even 

 among the more finely radiated forms. On some the ribs are flattened 



