SYNOPSIS OF GENERA. Ixi 



formations. The typical species of Pernopecien of Winchell, erroneously 

 included by Meek under Entolium, have a crenulated hinge-plate, although from 

 the nature of the matrix this feature is often obscure or concealed, and the 

 crenulations are easily abraded by friction. 



Permpeden should, in addition to the typical species, include those which 

 were afterward referred to Entolium, and properly exclude some of the New 

 York species of the Chemung group. 



Euchondria is "not well understood, although different from Auiculopeden 

 as usually characterized. At present it can only comprise the species described 

 as Aviculopeden negledus, Geinitz. 



CRENIPECTEN, Hall. 



PaL N. Y., vol. V, pt. 1. Plates and Explanations: p. 3. 1883. 

 [Type C. creniilatus, Hall.] 



Shell pectenoid in general form ; anterior ear the larger. Surface smooth 

 or radiated. Hinge furnished with a series of small, sub-equal, ligamental 

 pits or vertical teeth throughout its entire length. 



See plate ix, figs. 15 and 17 of C. crenulatus, and fig. 28 of C. WinchelU. 



This genus was instituted to include a number of species in the Chemung 

 and Waverly groups, which have a continuous row of sub-equal cartilage-pits 

 along the margin of the hinge. In Permpeden there is a ridge on each side 

 of the cartilage-pit, sloping downwards and following the course of the lower 

 side of the ears. Also the row of ligamental pits is not in a straight line, but 

 elevated towards the extremities of the ears, and it is interrupted by the well- 

 marked central cartilage-pit. 



The external form of several of the species referred to Crenipeden bears 

 considerable resemblance to some species of Streblopteria, but that genus has 

 a slender, oblique, posterior cardinal tooth, and the ligament is contained in a 

 simple narrow groove along the margin of the ears. 



