68 MOLLUSCA FROM THE CRAG. 



Locality. Cor. Crag, Passim. 



Red Crag, Passim. Var. |3, subobliquus, Walton Naze. 

 Mam. Crag, Thorpe, Bridlington (Leckenby). 



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. /3), 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 



versa, some are lenticular, with but little tumidity, others are much inflated. The 



exterior is generally more or less ornamented with raised, radiating, and distant striae, 



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 obtained 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. 



