514 GEOLOGICAL SURVEY OF IOWA. 



SURFACE marked by seven or eight plications on either 

 side of the mesial fold, and by nine on either side of the 

 mesial sinus, and these are crossed by fine imbricating la- 

 mellose striae : the entire surface granulose or punctate. 



This shell resembles the Cyrtia acutirostris described by Dr. SHUMARD in the 

 Geological Report of Missouri for 1855, pa. 204, pi c, f. 3 a, b, c; but the plications 

 are about twice as numerous : there is also no perceptible dilatation of the for.amen 

 near the base, as in that species. 



Geological formation and locality. In calcareous shale of the age of the 

 Hamilton group of New- York : Rock-island, Illinois. 



Pentaiiierus octidentalis ( n. s.). 



PLATE VI. FIG. 2 a, 6, c. 



SHELL ovoid, gibbous. Ventral valve arcuate, very gibbous 

 on the umbo, with the beak strongly incurved ; foramen 

 large and the cavity deep. Dorsal valve much smaller than 

 the ventral, moderately convex. 



SURFACE of the upper part of the shell smooth, or marked 

 only by concentric striae ; middle and lower part of shell 

 is marked by strong rounded or subangular plications, about 

 two, three or more of which are elevated upon the mesial 

 fold, the concentric striae being strongly arched in passing 

 over them. 



This species bears some resemblance to Pentamerus papilionensis of the Hamilton 

 group of New- York; but the beak of the ventral valve is more produced, and the 

 plications fewer and stronger. The specimen figured is an imperfect ventral valve. 



The Pentamerus is among the most rare of the fossils of the Hamilton group in 

 New-York : it is usual to meet with no more than one or two among a thousand 

 specimens of other fossils, and frequently a collection of several thousand specimens 

 has not shown a single Pentamerus. Although no very extensive collections have been 

 made in Iowa, we nevertheless observe that the Pentamerus is very rare, less than 

 half a dozen individuals having been observed in all the collections yet made. 



Fig. 2 a. Ventral view of an imperfect specimen. 



Fig. 2 b. Profile of the same. 



Fig. 2 c. Interior, showing the hinge, the triangular cavity, etc. 



Geological formation and locality. In calcareous shale and in limestone 

 of the age of the Hamilton group : Independence, ,Iowa. 



