INVERTEBRATES. 325 



It is impossible to confound this interesting shell, for a moment, with any 

 other known form of our entire Palaeozoic series. In young examples, as 

 already stated, it is much less convex, and presents somewhat the appearance 

 of an Orthis, its plications being at that stage of its growth rounded, very ob- 

 scure, and only indicated near the front. In larger specimens they are much 

 more prominent, and like the sinuses between, more angular, while the valves 

 are greatly more convex and more arched ; the beaks are also, in these larger 

 specimens, more incurved, and the area less apparent. 



Worn specimens, or internal casts of adult examples, sometimes resemble 

 some of the small globose species of the genus Pentamerus, particularly such 

 species as that described by Professor Hall {Iowa Report, vol. i, part ii, p. 514), 

 from the Hamilton group, under the name P. occidentalism but the more dis- 

 tinct costae and radiating striae readily distinguish our shell, where the surface 

 is intact. It is also a resupinate species, so that its dorsal valve corresponds 

 to the ventral, in the Pentamerus alluded to. 



In examining the striae under a magnifier, especially on the sides of the 

 valves, we notice that occasionally one of them increases slightly in size for a 

 short distance, to a point where there seems to be a minute opening, as if it 

 had been the base of a tubular hair-like spine. Immediately below each of 

 these the striae are each slightly less prominent. These little openings are 

 greatly larger and more scattering than the punctures of the substance of the 

 shell, which, however, are comparatively large, scattering, and easily seen with 

 a good pocket lens on exfoliated surfaces. 



Locality and position : Twelve miles north of Vandalia, Illinois; Upper Coal 

 Measures. This is the only locality at which this shell has been found in Illi- 

 nois. It occurs, however, at the same horizon in Eastern Kansas, Northern 

 Missouri and Western Iowa, where it is, as in this State, associated with nearly 

 all the common Upper Coal Measure species of the West. 



* As Prof. Hall had previously applied the name P. occidentalis to another species, 

 from Canada, in the second volume of the Palaeontology of New York, p. 341, it becomes 

 necessary, in order to prevent confusion, that our Hamilton group species of the West 

 should receive another name. We would therefore propose to call it Pentamerus gale- 

 atiform, from its resemblance to smooth specimens of P. galeatus. 



