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. In a 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 be 

 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. 



