BIVALVIA. 89 



This species in the recent state appears to be restricted to colder regions of the 

 northern hemisphere, and is essentially a Boreal species. In the Red Crag it is by no 

 means abundant, and until the discovery of the native bed at Chillesford, resting upon 

 the Red Crag, it was considered a shell of great rarity in our cabinets, but at this latter 

 locality it may now be obtained in considerable numbers, and the specimens have 

 there seemingly reached their full development in regard to size, some of mine having 

 a magnitude of two inches and a quarter in the longest diameter. This and My a 

 truncata are the most characteristic as well as the most abundant species in that 

 Deposit. The anterior side of the shell may be described as forming nearly half a 

 regular ellipsis, and the posterior side is smaller and rostrated ; the pointed termi- 

 nation curving a little upwards; a large lanceolated corselet or flattened space occupies 

 nearly the whole length of the posterior slope, and a smaller or more narrow one is 

 generally to be seen at the dorsal margin on the anterior side ; the number of teeth are 

 variable, generally about twenty on the posterior, and thirty on the anterior side, they 

 are large, prominent, sharp, and angular, with serrated edges, the lateral teeth 

 are more distant from each other than those nearer the umbo ; this is the 

 number in the adult shell ; in the younger state they are less numerous, additions 

 being made to their amount as the animal enlarges ; a sinus with two or three 

 obsolete rays curves over the anterior side at some little distance from the dorsal 

 margin, produced probably by the protrusion of a peculiarly formed foot at that part 

 of the edge of the shell ; coarse lines of growth cover the exterior, and these are 

 cut at a small angle by ridges which cross the shell in an oblique direction from 

 the anterior to the posterior ventral margin ; but not extending over the dorsal 

 portion of the shell on either side : in some aged specimens, the interior is irregu- 

 larly and ruggedly thickened, leaving deep impressions on those parts to which the 

 muscles of the animal were attached : that of the adductor on the anterior side is large 

 and angularly ovate, and not far from the extremity, the posterior one is smaller, and 

 situated more within the shell ; the curve formed by the retrocession of the siphonal 

 tubes is rather variable, extending in some specimens as far as the middle of the 

 ligamental area. The principal variation to which this species appears to be subject is 

 merely in the proportional dimensions. 



The name proposed by Mr. J. Sowerby has priority of date over that employed by 

 Lamarck for a very different species, which was published in 1819, and as such it 

 is retained here for the Crag Fossil ; and for the shell, therefore, so called by 

 Lamarck, I would substitute that of cultrata. 



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