26 MOLLUSCA FROM THE CRAG. 



Spec. Char. Testa mimitd, suborbiculatd, (Bquivalvi, sribaquilaterali, tenui, stMi/alind, 

 planulatd, fflabrd ; auriculis subecqiialibus, in vulvd dextrd antice longiori, rotundatd, postice 

 redangulatd ; sitiu hrevi, acufo. 



Shell small, suborbicular, equivalved, subcquilatcral, tliiu, subpcUucid, Hattencd, 

 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, .^Egcan 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. Loven as having been obtained on the coast of 

 Finmark, while Professor Forbes procured it in considerable numbers from a great depth 

 in the /Egcan Sea, some specimens of which he has obligingly given mc, for the pur- 

 pose of comparison with the British fossil. 



There arc some shght differences between the recent shell and the fossil, Ijut 

 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 iEgean, whicli were oljtained at the depth of 

 100 fathoms, are rather larger than any I have from the Crag, measuring a little 

 more than a cpiarter 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 lengtli. In 

 the right valve the anterior auricle is as large as the ])osterior 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 ; th(^ diverging line from 

 the umbo is on the j)osterior 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 Avith coloured markings of a zigzag, 

 or what is called Vandyke shape, traces of these colours are still remaining in some of 

 the Crag si)ecimens, one of which has only a single line of divergence from near the 

 centre, somewhat like the ornaments upon Lucina divuricata. There is also a slight 

 inequality in the dejith or convexity of the valves, the riglit or lower one being a little 

 more tumid than tlic other. 



