74 MOLLUSCA FROM THE CRAG. 
umbo to the ventral margin. The hinge is composed of five or occasionally six 
teeth in the right valve, with six or sometimes seven in the left, these are arranged 
in a slightly curved line extending on both sides of the beak, three on each side; 
those on the posterior or shorter side are placed rather closer together than those 
on the anterior one, they are prominent, obtuse, and large compared with the 
size of the shell; those in the centre standing nearly perpendicular to the line of 
hinge, while the outer ones on both sides have their widest portion in an opposite 
direction, all placed, of course, so as one set can be interposed between those of 
the opposite, when the valves are closed. The lateral tooth of the right valve 
has a deep depression between it and the margin for the reception of a large tooth 
of the left valve. Ina specimen of the French Eocene Fossil in my own cabinet, the 
teeth do not appear to be quite so obtuse as in the Crag shell, and the umbo is somewhat 
sharper and more terminal, with a rather more angular outline, the posterior side is 
apparently more truncated or straight, while the shell is nearly transparent. 
A species passing out of one Period, where the animals or the remains of them, are 
of a nature to indicate conditions differing materially from those of another Period into 
which it is supposed to have had its existence prolonged, and so far removed as 
to have almost its entire Fauna formed upon a different type, would in all pro- 
bability be affected in some degree by the change, so as to produce a slight alteration 
in its calcareous covering, such as would constitute what is called a variety, and these 
characters might become permanent under the continuance of those altered conditions ; 
but should we not be permitted so to undermine specific integrity, we may at least he — 
allowed a latitude in variation, that is ordinarily conceded to the examination of 
existing forms, and the differences between the shells of the two periods, which are 
here considered as identical, is less than is oftentimes presented to us by individuals 
of undoubtedly the same species in the Crag deposits; even in those recent species 
that appear to be admitted by almost general consent, as having originated in the 
earliest Tertiary Periods, a difference may be detected between the older and the more 
recent specimens, showing those animals that are apparently possessed of capabilities 
of endurance beyond their contemporaries, have not been able to maintain in strict 
integrity the supposed unvarying characters originally impressed upon them; all, how- 
ever, that is contended for here is, that no greater restriction in regard to the limits of 
variation ought to be imposed upon the line of specific demarkation, merely from 
differences in Geological Periods, than is granted to deviations among specimens from 
the same deposit. 
