GASTEROPODA. 29 



have distinctly marked characters ; but it is not possible to restrict the variety 

 to such forms, unless by indicating as distinct varieties several of the other 

 forms illustrated. 



Formation and locality. In limestone of the Upper Helderberg group, in 

 Onondaga, Onondaga county. 



Until the discovery of this locality by Prof. E. A. Strong, few individuals of 

 the type of P. turbinata were known, and these were very constant in their 

 characters. All the varieties of form represented in the illustration of this 

 species have been derived from that locality, which has afforded probably 

 twenty times as many specimens of the species as all other localities in the 

 State. Strophostylus varians is not known to me from any other locality; 

 and Platyostoma lineata, and several species of Platyceras, are also common in 

 the same place. 



It would appear that the physical conditions favoring the abundant pro- 

 duction of individuals has, at the same time, favored a degree of variation 

 unknown under conditions existing elsewhere. 



Platyostoma turbinata var. 



PLATE IX, FIGS. 27-30. 

 Platyosttrma turbinata var., Hall. Illustrations of Devonian Fossils : Gasteropoda, pi. 10. 1876. 



I have indicated in the Illustrations of Devonian Fossils, pi. 10, the forms 

 above cited as a variety of P. turbinata, the principal features of which are the 

 low turbinate form, depressed spire, aperture narrow and extended vertically, 

 with a distinct sinus in the upper margin of the peristome. 



In this variety the spire is more elevated than in typical forms of P. turbi- 

 tfatOj I mt does not rise so high as in V. cochleata. Compared with the latter, the 

 volutions are less rotund, obscurely angulated at the periphery, with a very 

 similar form of aperture. The specimen (figs. 28-30) is from the Hamilton 

 group in the neighborhood of Moscow, N. Y. Its surface, as shown in fig. 28^ 

 Lb incrnsted by a Bryozoan. 



