SYNOPSIS OF GENERA. 
lxi 
formations. The typical species of Pernopeden 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. 
Pernopeden 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. 
[Tjrpe C. crenulatus, 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. Winchelli. 
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 Pernopeden 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. 
