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 Falls; in the coarser shales at Cazenovia in Madison county, and at 
F ultonliam in Schoharie county, N. Y. 
Cyrtolites (Cyrtonella) pileolus. 
PLATE XXV, FIGS. 20-22, 29 ; and PLATE XXVI, FIG. 25. 
Cyrtolites 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. 
9 
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 
