MYA. 71 



very long, subhyaline, and delicately frosted : gills pale brown ; 

 lower one of each pair much larger than the other, lying hori- 

 zontally, and obliquely overlapped by the upper one: palps 

 somewhat triangular and pointed : foot small, narrow, subcy- 

 lindrical, of a bluish transparent hue ; it produces a byssus of 

 a few coarse filaments. 



SHELL wedge-shaped, decidedly inequivalve and inequi- 

 lateral, gaping at the posterior end, but not to the same extent 

 as the young of M. truncata, compressed, rather solid, opaque, 

 and somewhat glossy : sculpture, numerous fine but irregular 

 concentric striae, and occasional stronger marks of growth: 

 colour milk-white under the epidermis, which has a brownish- 

 yellow cast, and is often strongly wrinkled on the posterior 

 side, and extends over part of the pallial sheath : margins 

 obliquely truncated on the anterior side, usually straight in 

 front, and narrowing to an abrupt and straight edge on the 

 posterior side ; this latter part has in each valve a blunt angle 

 or keel running diagonally from the beak to the lower point 

 of the posterior extremity ; dorsal margins extremely short on 

 the anterior side, long and mostly straight (although sloping) 

 on the opposite side : beaks small, incurved, not contiguous : 

 cartilage yellowish-brown : hinge-line slightly curved : hinge- 

 plate narrow : teeth, in the right valve a small and blunt but 

 distinct cardinal, besides the cartilage -pit, which is placed as 

 usual in this genus ; in the left valve the erect triangular tooth 

 is flatter and less elevated than in the preceding two species, 

 and considerably elongated on the posterior side : inside por- 

 cellanous : muscular scars extremely large, and placed near the 

 edges of the shell. L. 0-25. B. 0-5. 



Var. elongata. Shell considerably broader in proportion to 

 its length, which is nearly equal throughout and gives a cylin- 

 drical appearance ; posterior dorsal margin sometimes concave 

 and turned up at the extremity. 



HABITAT : In the cavities of limestone rocks and old 

 oyster-shells perforated by Saxicava rugosa and Cliona 

 celata, as well as among the roots or bases of Laminaria 

 saccharina, and in other places of shelter and conceal- 

 ment ; Channel Isles northward to Scarborough (Bean) 

 and Skye (Barlee), and all the coast of Ireland, in 

 5-25 f. The variety is found in the deserted cases of 



