OF DENISON DNIVERSn'Y. 
23 
coal-measure species, many being similar, it is hazardous to identify it 
certainly with any of them. ^ 
LAMELLIBRANCHIATA. 
Entoliuin aviculatums Swallow. 
(Plate I, Figs, i, 2 ; Plate III, Fig. jo.) 
It is submitted that it is impossible to rescue the Fntoliums of the 
carboniferous from the inextricable confusion into which they have 
fallen. Nevertheless the name F. aviculatum doubtless applies to a 
species of wide distribution in the coal-measure rocks, however it may 
be related to sub-carboniferous species. 
The remarks of Prof. Meek, under his description of an Fntolium 
from the Waverly, are instructive as showing the difficulty of discrimi- 
nating species in this group. He also says Prof. WinchelPs type 
[of Pernopecten shumardianus] seems to agree exactly, in all constant 
specific characters, with Entolium aviculatum. Swallow, already mentioned, 
from the coal-measures. So we have here a remarkable case of shells 
presenting extremely little or no constant specific difference, and yet 
differing in a character of the hinge that seems to be of generic impor- 
tance.” 
Although Prof. Hall figures Pernopecten shumardianus from New- 
ark, O., we have found no such form, though another and smaller 
shell apparently belonging to Pernopecten, is rarely found in the Wa- 
verly. We thought for some time that by mistake some Flint Ridge 
or Bald Hill specimens (coal-measures) had been labeled WNverly, but 
none of our numerous specimens from the coal-measure shales show 
the crenulated hinge prominences ascribed to the species in question. 
Mr. Meek, however, supposed P. shumardianus to be without the 
crenulated hinge and others have decided that Fntolium is synonymous 
with Pernopecten. If that be the case it is difficult to see how F. 
aviculatum can be distinguished from Pernopecten shumardianus. 
The hinge in all specimens seen from the coal-measures shows, instead 
of a prominence, an impressed line on the left valve, at the point des- 
ignated in Penopecten by a crenulated ridge, this appears on the out- 
side of the valve as a ridge, the shell being very thin. On the ventral 
valve in the same position is a ridge, designed, it 'would seem, to fit 
into the groove of the other valve. The left valve, however, has its 
