NIAGARA GROUP, 



287 



649. 2. PLATYOSTOMA NIAGARENSIS (n. 5;?.)- 

 Pi.. LX. Fig. 1 a-v. 

 Compare Nerita haliotis, Sow. in Murchison, Sil. System, pag. 625, pi. 12, fig. 15. 

 Globose ; volutions three or four ; body-whorl large, inflated towards the aperture which is 

 dilated ; sutures deep ; spire depressed (rarely elevated) ; shell thin ; surface striated across 

 the volutions, and in well preserved specimens longitudinally marked by filiform undulating 

 striae. 



The spire appears to be depressed often when the shell retains its natural proportions, and 

 at other times from pressure ; in a few examples, it is considerably elevated. The fine un- 

 dulating longitudinal strias do not always appear, and sometimes only upon a portion of the 

 surface, even when there is no appearance of abrasion. In other examples they have evidently 

 been worn off, leaving the transverse striae well preserved. 



Fig. 1 a, b, c. Young individuals, where the spire is depressed as low as the outer volution. 

 These specimens show only transverse strias. 



Fig. 1 d. An individual having the spire equally depressed, and the surface conspicuously marked 

 by longitudinal strias. 



Fig. 1 e. Profile view, showing the depressed spire. 



Fig. 1 /. A small specimen having an elevated spire, with the surface marked as in tig. 1 d. 



Fig. 1 g. View looking on the spire of a similar specimen. 



Fig. 1 h. A small specimen having the spire much elevated, and the x'olutions longitudinally 

 and transversely striated. 



Fig. 1 /, k, I. Back, summit and front views of a globose specimen preserving its natural pro- 

 portions. The spire is moderately elevated, the volutions very rotund, and the aperture 

 tolerably well preserved. The surface is strongly marked by transverse striee and 

 faint indications of longitudinal ones. On the middle of the last whorl there is an 

 elevated band reaching half way to the aperture, beyond which the surface is plainly 

 striated. 



Fig. 1 m, n. Two views of a similar specimen, having the aperture somewhat more expanded. 



Fig. 1 0, p. Two views of a larger individual with an expanded aperture and a strong fold upon 

 the back of the volution, and a sinus in the margin of the aperture. 



Fig. 1 r. A similar larger specimen. 



Fig. 1 s, t. View of spire, and partial front view of a large specimen from which the shell is 

 partially exfoliated. In this one there is no evidence of a band on the middle of the 

 volution, and but a slight arching backwards of the strice. 



Fig. 1 u, V. View of the summit of two other individuals having the usual depressed spire of 

 this species. 



Position and localities. This species occurs at Lockport, Rochester, Wolcott, and other 

 places where the shale is exposed. It is never abundant, though occurring in nearly all localities. 



