F. B. Meek—Fossils of the Illinois Geological Report. 581 
In subsequently studying the Illinois Coal-measure fossils, 
however, I soon found that there is there, even in the lower 
art of the Coal-series, a slightly variable shell, agreeing well 
with the Nebraska City species, and yet readily distinguishable 
by its constantly more compressed form, and less prominent 
beaks, from S. curtus. Being, at the time the plates were pre- 
pared, undecided what disposition to make of these more com- 
pressed forms, I merely wrote, temporarily, the name S. Rossicus 
plate, intending ultimately to study them more carefully, be- 
ore coming to a final conclusion in regard to their relations. 
The opportunity to do so, however, was prevented, as elsewhere 
explained, and the name printed in the explanations of the plate 
as at first provisionally written. 
That these shells are really identical, however, with the 
Russian Permian species S Aossicus, is, I think, extremely 
doubtful; though they nearly resemble that species in form; 
and there is, according to Hichwald, a shell sometimes found, in 
the Carboniferous rocks of Russia, that he could not distinguish 
from S. Rossicus. Still, in a genus like this, where the species 
often closely resemble each other, and present few external 
distinguishing characters, excepting the often variable one of 
mere form, it is perhaps nearly, if not quite, impossible, in many 
cases, to determine beyond doubt the exact relations between 
closely allied forms, until their hinges and interiors can be care- 
fully compared. he 
While looking over the Illinois collections, I was fortunate 
enough to find two specimens of the compre orm repre- 
sented by our figures (see fig. 17c, d), showing the hinges of 
both valves very clearly. An examination of these fi ures 
will show that this shell presents some rather marked differ- 
ences in the characters of its hinge, from the typical species of 
Schizodus, as illustrated by Prof. King. That is, it has but two 
true teeth in each valve, instead of three in one and two in the 
other; while the one corresponding to the deeply bifid or split 
tooth in the typical Schizodus is only a little emarginate. Ac- 
cording to Prof. King, however, the teeth of this genus are 
subject to some variations, while Prof. McCoy, who has care- 
fully studied the hinges of the British typical species of the 
“pa (some of which seem not to have been seen by Pro 
