26 MOLLUSCA FROM THE CRAG. 
Spec. Char. Testé minutd, suborbiculatd, aquivalvi, subequilaterali, tenui, subhyalind, 
planulatd, glabré ; auriculis subequalibus, in valvd deatra antice longiori, rotundatd, posticé 
rectangulata ; sinu brevi, acuto. 
Shell small, suborbicular, equivalved, subequilateral, thin, subpellucid, flattened, 
smooth ; auricles nearly equal; the anterior one of the right valve rather the longer 
and rounded, on the posterior side it forms a right-angled triangle, with a short and 
acute sinus. 
Diameter, } inch. 
Locality. Cor. Crag, Sutton. Recent Britain, Finmark, Aigean Sea. 
This pretty little shell is particularly abundant at Sutton, in the Coralline Crag, to 
which formation, as far as is at present known, it is restricted: its minuteness and 
fragility may, however, be one cause why it has not yet been found in the Red Crag, 
as it appears to be a species possessed of capabilities to endure a considerable range in 
temperature, being quoted by Dr. Lovén as having been obtained on the coast of 
Finmark, while Professor Forbes procured it in considerable numbers from a great depth 
in the Aigean Sea, some specimens of which he has obligingly given me, for the pur- 
pose of comparison with the British fossil. 
There are some slight differences between the recent shell and the fossil, but 
which can hardly be considered of sufficient importance to justify it being ranked 
as more than a variety; the living species in several characters is subject to 
variation. The specimens from the Aigean, which were obtained at the depth of 
100 fathoms, are rather larger than any I have from the Crag, measuring a little 
more than a quarter of an inch in its longitudinal diameter, and a trifle less in 
the height, with the auricles unequal, the anterior one, more especially in the 
right valve, less than the posterior, but this is not a permanent character, as in 
some specimens they are equal in size. The Crag shell does not exceed in 
diameter a quarter of an inch, very rarely attains this magnitude, and the dorsal 
margin, or rather the ears of the shell, extend to 5-6ths of its entire length. In 
the right valve the anterior auricle is as large as the posterior one, and completely 
rounded with a small sharp sinus beneath it, as if the shell, in the living state, had been 
supplied with a byssus. There is a slight peculiarity in this species in the right valve, 
attributable probably to the presence and size of the byssus ; the diverging line from 
the umbo is on the posterior side rather convex, while on the opposite, or beneath the 
projecting ear, it is distinctly concave ; in the left valve the auricles are equal in size, 
and the divergence of the edge of the shell more regular, forming an angle of 90°; 
this valve is, in the recent state, ornamented with coloured markings of a zigzag, 
or what is called Vandyke shape, traces of these colours are still remaining in some of 
the Crag specimens, one of which has only a single line of divergence from near the 
centre, somewhat like the ornaments upon Lwcina divaricata. There is also a slight 
inequality in the depth or convexity of the valves, the right or lower one being a little 
more tumid than the other. 
