68 MOLLUSCA FROM THE CRAG. 
Locality. Cor. Crag, Passim. 
Red Crag, Passim.—Var. (3, subobliquus, Walton Naze. 
Mam. Crag, Thorpe, Bridlington (Lechenby). 
Recent, Britain, and Mediterranean. 
This is one of the most common and abundant shells in the Coralline as well 
as in the Red Crag Deposits. In the Coralline, the valves, as might be expected, are 
often found united. 
The determination of this species is exceedingly difficult, and the form which was 
figured in the ‘ Mag. Nat. Hist.’ (var. 8), presented characters it was then thought 
sufficient for the establishment of a new one, but the recent species has been found to 
exhibit the same obliquity ; this variety I have never seen from the Older or Coralline 
Crag, but it is one of the commonest shells at Walton on the Naze, where the two 
valves are frequently found united; it is generally thinner, and some specimens 
are very oblique, and this may be considered the limit of range in variation in 
one direction ; var. a in the other; between these forms every imaginable gradation 
may be pointed out in almost any collection possessing a good series of this abundant 
Crag shell, so well named by Mr. J. Sowerby (variabilis). There is scarcely a 
possibility of giving a correct diagnosis of this species, but what some deviation may 
be pointed out, and in consequence of which the varieties have been made into 
several species, as may be seen in the above list of synonyma, all, it is presumed, 
belong to this species. Specimens are somtimes longer than they are broad, and vice 
versd, some are lenticular, with but little tumidity, others are much inflated. The 
exterior is generally more or less ornamented with raised, radiating, and distant strie, 
variable in number, producing a like variation in the number of crenulations upon the 
interior margin of the valves ; in some they are as many as sixty, while in others they 
do not exceed thirty-five; neither is the number of teeth or denticles of the hinge a 
more permanent character, for in old specimens the ligamental area is pushed so 
far forward as to have obliterated all the central teeth, and they become almost 
toothless, not more than three or four remaining; while in some specimens as many as 
eighteen may be counted on each side of the umbo, they are prominent, somewhat 
angular, flattened on the top, and when perfect, generally crenulated on the edges ; 
between each is a deep depression for the reception of those in the opposing valve, 
and in very young shells the hinge is almost entirely destitute of denticles (fig. 1, e). 
Every size may be readily obtained, and my cabinet contains a series varying from 
specimens less than the eighth of an inch to those in which the diameter is nearly 
three and a half inches, dimensions exceeding those generally obtamed in our seas ; 
and this magnitude may be seen in shells from the Coralline as well as the Red Crag 
Deposits, while the species seems to have been rare in the Norwich beds. 
Some American Tertiary shells figured by Conrad under two or three different 
names, approach so closely to those of the Crag as to render the distinction doubtful, 
as far as regards representation alone. 
