15 



wavy ridges, which continue directly across the shell from the 

 posterior half, where they are most distinct, to the anterior half ; 

 as they go directly across the dome, they have an arched appear- 

 ance, if viewed from the side ; they are the most prominent on 

 the left side. 



The appressed, not spiral apex, and fine transverse arched, instead 

 of radiate, plications are the most distinguishing characteristics 

 which separate this species from the other campanulate forms. 



PLATYCERAS MEMBRANACEUM (N. Sp.). PI. 2, fig. 3. 



Shell very thin, long, conical, rapidly tapering near the upper 

 part to a sub-lateral apex. Aperture with a few, small, dentate 

 projections, placed far apart, and would be somewhat oval if the 

 specimen were uncompressed. Apex unknown, but judging from 

 the contour it was sub-acute. Surface not plicate, with a few, faint 

 traces of concentric striae. 



This species resembles P. pUcatum, Hall, in form ; but differs 

 in the exceedingly thin shell being without plications or cancel- 

 lations. 



The necessity for a revision of the genus Platyceras will forci- 

 bly present itself upon the grouping of such distinct and widely 

 divergent forms as the three here described, and is more evident 

 than when examining the forms from a group rich in species, 

 e. g., those from the Delthyris Shaly Limestone ; figured jn the 

 Paleontology of N. Y., Vol. 3, where they can readily be graded 

 from one to the other. A further division, however, at least into 

 sub-genera, would be very convenient, and avoid confusion. 



PENTAMERELLA COMPRESSA (N. Sp.). PL 2, fig. 4. 



Shell ventricose, laterally compressed, length one-third greater 

 than width ; strongly plicate, with a prominent mesial fold. 



Ventral valve with beak acute, incurved, and projecting over 

 that of the dorsal valve ; area on the sides of the fissure well 

 defined and high, continuing to the apex. Foramen elongate, 

 narrow, mesial sinus deep, and projecting much beyond the 

 others. 



