GASTEROPODA. 125 



regularly rounded upon the back, while it is obscurely angular along the 

 dorsal line. 



Notwithstanding the wide difference presented in the surface-markings of 

 the two forms, as illustrated in figs. 28 and 29 of plate 25, I conceive it to be 

 possible that the variations observed in the form, and the much greater varia- 

 tion in the surface sculpture, may be due to conditions of growth and to other 

 causes operating upon the shell to develop one set of striae in excess of the 

 other, and to such a degree that one may become obsolete while the other 

 predominates. 



Formation and localities. In the calcareous layers of the Hamilton group at 

 Pratt's Fulls; in the coarser shales at Cazenovia in Madison county, and at 

 Fultonham in Schoharie county, N. Y. 



Cyrtolites (Cyrtonella) pileolus. 



PLATE XXV, FIGS. 20-22, 29 ; and PLATE XXVI, FIG. 25. 



OgrtoUiM pileolus, Hall. Descriptions of New Species of Fossils, etc., p. 33. 1861. 



" Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 61, pi. 6, figs. 10, 11. 1862. 



" mitella, " Illustrations of Devonian Fossils : Gasteropoda, pi. 25, figs. 13-15 (not 16-21). 1876. 



Not " pileolus, " Illustrations of Devonian Fossils : Gasteropoda, pi. 25, figs. 22-24. 



Shell obliquely or arcuately subconical. Apex of the spire making a single 

 slender volution, and rapidly spreading below; more or less distinctly 

 carinate or angular on the back. Aperture nearly round, a little angular 

 in front ; peristome expanded, without apparent sinus, or with a slight 

 depression in the anterior margin. 



Surface marked by distinct, undulated revolving strife, which become less 

 conspicuous towards the aperture; obscure striae of growth mark the 

 surface transversely, and these become more distinct in the casts of the 

 fossil and towards the margin. 



In this species the sides of the volution are usually less abruptly expanded, 

 And the dorsum is sometimes less distinctly angular (or subcarinate) than 

 in the preceding species. The equal, undulating, revolving striae constitute 



