AST ARTE. 457 



land, is such that it becomes very difficult to draw a line 

 between recent and fossil examples of many of our boreal 

 species. This difficulty is greatly increased when the spe- 

 cimens are jirocured from a considerable depth of water. 

 The shell we have now to describe has only been taken in 

 a state much more ancient to appearance than most of our 

 pleistocene Astartes, yet we prefer describing it as an ex- 

 isting form, on account of the locality in which it was first 

 procured presenting no traces of the vicinity of pleistocene 

 strata, and because no fossil examples of it have ever oc- 

 curred in beds of that age, which having been upheaved, 

 have undergone close scrutiny. Only odd valves of this 

 curious species have been taken ; these valves are subtri- 

 angular, longer than broad, solid, opaque, and rather com- 

 pressed ; they are destitute of any epidermis, are of a dirty 

 or jmle reddish-white colour; look slightly porous, and 

 feel harsh to the touch. There is not the least appearance 

 of lustre ; and their only sculpture consists of very nume- 

 rous (at the least thirty) and crowded concentric ribs, which 

 are sharply defined, extremely narrow, and extend the 

 entire distance from the beaks to the ventral margin, but 

 become obsolete upon and beyond the ordinary site of an 

 umbonal ridge. The interstices are simjjle, that is to say, 

 are not traversed by either longitudinal or transverse striae, 

 and are for the most part rectangularly grooved out, the 

 edges of the ribs being usually but little, if at all, shelving. 

 The sides are unequal, but not particularly so ; the hinder, 

 which is the longer, has its termination somewhat at- 

 tenuated, and obtusely sub-biangulated, and its tip sub- 

 rectilinear, or scarcely convex ; the front extremity is 

 rounded, but not symmetrically, as the arch of its lower 

 portion is far more bowed out than its upper outline. The 

 declination of the dorsal edge (a character of little value in 



VOL. I. 3 N 



