SAXICAVA. 147 



scarcely at all glossy, paler or darker, brownish yellow 

 epidermis, beneath which the surface is utterly devoid of 

 lustre, and very coarsely wrinkled in a concentric direction. 

 From the beaks to either side of the posterior extremity, runs 

 a more or less obsolete elevated line, which is jnnckled with 

 small and rather elongated vaulted scales, but only entirely 

 so in the very young, and partially so (in the vicinity of 

 the beaks) in the middle-aged specimens, the full grown aged 

 examples rarely presenting the slightest trace of their pre- 

 vious existence. Both the beaks, which are simply incurved, 

 and the umbones, are very far from prominent, and exhibit 

 no trace of a lunule in front of them, the lips of the anterior 

 dorsal edge more frequently on the contrary protruding 

 outwards. The front side is invariably much shorter than 

 the hinder one, which is at the least twice and a half its 

 length, but its proportion to the entire area is generally 

 much greater than in arctica, owing to the anterior ex- 

 tremity being typically round and never sharply cuneiform, 

 the curve of the anterior outline sweeping outwards, not 

 bending inwards. The ventral edge is generally inclined 

 to retusion a little before the middle, and runs nearly 

 parallel to the hinder dorsal, which latter, however retuse 

 it may be near the beaks, always becomes convex before 

 its termination. The hinder extremity is more or less ob- 

 tusely subbiangulated, but never so distinctly and sharply 

 as in arctica ; moreover, it is always, when freely de- 

 veloped, disposed to taper a little just before its termination, 

 and in one narrow variety which has its posterior side 

 greatly produced, and its anterior one correspondingly 

 abbreviated, this attenuation is most distinctly visible. 

 The ligament is brown, (yellowish or ashy broAvn in the 

 young,) and in typical examples is prominent and forms 

 one piece with the epidermis, but these characters are rarely 



