84 
PAL2EONTOLOGY OF NEW YORK. 
volution both the concentric and revolving striae become strong and coarse, 
the latter in fascicles, and where crossing the strong transverse striae pro¬ 
duce a nodose surface, giving the shell a coarse, rude aspect, quite unlike 
the 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 striae are subdued or obsolete 
and the concentric striae become strongly developed, the shell has the aspect 
of P. arata. 
Formation and locality. In the shales of the Hamilton group near Skaneateles 
lake and at Bcllona, Yates county, N. Y. 
Pleurotomaria disjuncta. 
PLATE XXI, FIG. 18. 
Pleurutomarla disjuncta, Hai,l. Illustrations of Devonian Fossils: Gasteropoda, pi. 20, fig. 32. 1876. 
Shell depressed-trocliiform; sjfire 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 carinae, 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 striae. The peripheral band is 
a simple concave sjiace with curving striae. The concentric striae 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 striae 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 striae they become a little nodulose. 
