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 JJ/ya 
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; im 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 cu/trata. 
