4 PALEONTOLOGY OF NEW YORK. 



Surface marked by concentric undulating striae, which become sublamelloae 

 towards tin- aperture, and are sometimes closely crowded and wrinkled 

 with numerous knots or nodes. Peristome deeply sinuous; the width 

 from the anterior to the posterior side a little greater than the transverse 

 diameter. The length of the shell is one inch and a half or more, with 

 the aperture a little less. 

 Tbis species approaches the P. pyramidatum of the Lower Helderberg group, 



but is less elongate, the peristome is more sinuous, and the indication of lon.iri- 



tudind ridges and depressions is more, distinct; the crowded, wrinkled and 



nodose stria? are likewise a distinctive feature. 



Formations and localities. In the Hamilton group, Ontario county ; and in the 



Upper Helderberg limestone, at Darien and Williamsville, N. Y. 



Platyceras (Orthonychia) perplexum. 



plate ii, figs. is. 



Platyceras perplexum Hall. Illustrations of Devonian Fossils : Gasteropoda, pi. 2. 1876. 



Shell obliquely conical, apex minute, closely incurved, or making a part of one 

 volution. The body-volution slightly curved, expanding gradually from 

 above to the middle of the shell, below which it spreads more rapidly 

 upon one side, becoming strongly plicated ; the plications beginning 

 above the middle of the shell, and sometimes at a short distance below 

 the apex, and increasing in number and strength towards the base. 

 Aperture subovate ; peristome deeply sinuate. 

 Surface marked by close concentric stria) of growth which become lamellose, 

 and crowded into undulating bands crossing the radiating ridges. 

 This is an extremely rare form, and is quite distinct from any of the other 

 species ; although presenting in the form of the body-volution an approach to 

 some varieties of P. Thetis, it has not the same degree of arcuation, and has 

 never the distinctly convoluted nucleus. 



This species bears some resemblance to P. perplicatum of the Lower Helder- 

 berg group; but a comparison of specimens shows them to be quite distinct. 



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

 Onondaga county, N. Y. Communicated by Prof. E. A. Strong. 



