NATURAL SCIENCES OF PHILABELPHIA. 1G9 



hinge, 1 inch. Another specimen, 1.54 inches in length, has a 

 convexity of 0.15 inch. 



It is barely possible that this may be a variety of the last, bnt 

 as I have before me ten good specimens of that shell, and two of 

 the form under consideration, and there are among them no inter- 

 mediate gradations between the two forms, I can but regard them 

 as distinct species. The shell here described differs from the last, 

 with which it was found associated, in being proportionally 

 decidedly shorter and wider (higher), as well as in having its ven- 

 tral margin much more prominent or deeply rounded in the cen- 

 tral region. Its beaks are also less oblique, rather more promi- 

 nent, and proportionally farther from the anterior end. It shows 

 some faint traces of a similar anterior oblique umbonal ridge to 

 that seen in the preceding species, but it is less distinct, and does 

 not show so decided a tendency to become angular at the beaks. 



Locality and position. Rushville, Ohio. Waverley group. 



PLATYOSTOMAT TRIGONOSTOMA, Meek. 



Shell strongly depressed or subdiscoid, with the periphery an- 

 gular ; spire so low that the shell is less convex above than below 

 the periphery ; volutions three, XQvy rapidly increasing in size, 

 particularly in breadth, merely' with an outward slope above ; last 

 one large and compressed convex, but not much projecting below, 

 a little declining near the aperture on the inner side above ; suture 

 linear ; aperture large, subtrigonal, with breadth greater than 

 height ; lip extended forward on the inner side above, and appa- 

 rently very oblique. (Surface marking unknown.) 



Breadth 1.35 inches ; height about 0.60 inch. 



I am not sure that this is a true Platyostoma. It is far more 

 depressed in form than anj- of the described species of that genus, 

 and, judging from some faint undulations on the cast apparently 

 corresponding to the direction of the lines of growth on the upper 

 and lower sides of the body volution, these lines would seem to 

 have curved strongly backward in passing outward toward the 

 peripher}^, thus indicating the presence of a rather deep, broad 

 sinuosit}' of the lip at the termination of the peripheral angle. If 

 this is the real direction of the lines of growth, it would probably 

 be nearer correct to call the species Pleurotomaria trigonostoma ; 

 but as there is no appearance of a band on the periphery, and the 

 lines of growth are not certainly known to describe these curves, 

 1871.] . PART II.— 12 



