70 PA LAVONTOL 00 V OF NE W YOR K. 



This is the most common species of Plecrotomaria in the Hamilton group, 

 occurring in all places east of Seneca lake, and often abundant in the coarser 

 shales of Madison county. To the west of Seneca lake it is very restricted in 

 its vortical range, and is comparatively rare. The form is usually depressed- 

 trochiform, although sometimes it attains an elevation of seven-eighths of an 

 inch, with a diameter of one inch. The retral bending of the sharp concentric 

 strise, the distinct carina just above the suture on the upper volutions, and the 

 concave spiral band on the angular periphery, are distinguishing features. 

 The casts are, however, often rounded or obtusely subangular on the periphery : 

 some specimens in this condition from Maryland measure one inch and a half 

 in diameter, and one inch and three-eighths in height, and consist of about five 

 distinct volutions. 



Formations and localities. This species has been found in limestone of the 

 Upper Helderberg group, at the Falls of the Ohio. In the State of New York 

 it occurs in the Hamilton group at Delphi, Pratt's Falls, at Pompey Hill and 

 other places in Onondaga county — at the two former places more abundantly 

 than elsewhere in the State. It also occurs in numerous localities in 

 Madison and Otsego counties, and less frequently in Schoharie county. It is 

 often found on the shores of Seneca and Cayuga lakes, but rarely farther west- 

 ward. Its favorite habitat has been in semi-arenaceous sediments of the central 

 portion of the group, with conditions intermediate between the arenaceous 

 shales of the east and the soft calcareo-argillaceous muds of the western part of 

 the State and Canada. In similar association, this species extends in a south- 

 westerly direction to Maryland and Virginia. 



Pledrotomaria delicatula. 



PLATE XIX, FIGS. 18, 19. 

 Pleurotomaria delicatula, Hall. Illustrations of Devonian Fossils : Gasteropoda, pi. 19. 1876. 



Shell trochiform ; spire depressed-conical, moderately elevated. Volutions 

 four or more, gradually enlarging from the apex, the last one becoming 

 ventricose towards the aperture, which is somewhat rhomboidal. 



