S J PA L^EONTOLOG Y OF NE W YORK. 



volution both the concentric and revolving striae become strong and coarse, 

 the latter in fascicles, and where crossing the strong transverse stria) pro- 

 duce a nodose surface, giving the shell a coarse, rude aspect, quite unlike 

 tin' ordinary forms of the species, and especially differing from the typical 

 character of those in the limestone of the Upper Helderberg group, as 

 illustrated in figures 8 and 9 of plate 18. Several other specimens possessing 

 similar characters, in a less degree, have been observed. In these modifications 

 of the surface markings, where the revolving stria) are subdued or obsolete 

 and the concentric stria) become strongly developed, the shell has the aspect 

 of P. arata. 



Formation and locality. In the shales of the Hamilton group near Skarieateles 

 lake and at Bellona, Yates county, N. Y. 



Pleurotomaria disjuncta. 



PLATE XXI, FIG. 18. 



Pleurotomaria dwjuncta, Hall. Illustrations of Devonian Fossils : Gasteropoda, pi. 20, fig. 32. 1876. 



Shell depressed-trochiform ; spire moderately elevated. Volutions (number 

 unknown) subangular on the upper side, and rounded below, the last one 

 moderately ventricose. The form of aperture not fully known. 



Surface on the upper side marked by revolving carina), the first of which is 

 nearly midway between the suture and the periphery, beyond which are 

 two others more conspicuous than the inner one. Below the periphery 

 are numerous strong revolving cariniform stria). The peripheral band is 

 a simple concave space with curving striae. The concentric stria) are 

 acute, strongly defined and directed backward from the suture, scarcely 

 changing their direction in passing the first carinate band, but sometimes 

 become a little nodulose; after crossing the second ridge the stria) make a 

 more abrupt retral bend, and curving over a narrow carina to the peripheral 

 band, where they are much subdued, are continued on the lower side in 

 the same strength as above, directed slightly forward. In crossing the 

 stronger revolving stria) they become a little nodulose. 



