342 DONACID^. 



prominent external ligament), is oblong elliptic, its length 

 being nearly double its bi-eadtli. It is solid, opaque, and 

 tolerably convex, the swell diminishing moderately and 

 evenly from the umbones on either side. The colouring 

 is slightly variable, but always of darker or lighter tints 

 of Avarm chestnut, the ground-colour being usually pale 

 or almost whitish (but sometimes decided chestnut), with 

 an almost vertical ray of dark chestnut-brown, which is 

 usually narrow, and does not reach to the beaks, lying 

 almost directly beneath them, and very slightly curving to 

 the longer side : the extreme posterior portion is likewise 

 stained with a similar brown tinge, forming a kind of 

 obsolete broad ray externally, but usually displayed more 

 distinctly upon the interior ; there is often too a third but 

 almost obsolete ray-like stain adjacent to the anterior 

 extremity. The surface is slightly glossed, and, from the 

 more or less abraded state of our English specimens, is 

 quite smooth ; in the living examples, however, it is con- 

 centrically substriolate, the striulaj being more closely and 

 regularly disposed on the anterior side. The ventral mar- 

 gin is convex or subarcuated, and rises the more anteriorly, 

 where it often forms a slight subangulation above with the 

 dorsal edge, which margin is more or less rectilinear towards 

 the beaks, but subsequently has a tendency to display 

 convexity. The produced posterior side is attenuated and 

 bluntly rounded at its termination ; the anterior varies 

 in these respects, but is more usually narrowed and only 

 rounded below, yet occasionally is rather the broader eiul, 

 and rounded both above and below ; the hinder dorsal 

 margin is retuse near the beaks, then straightish, and 

 finally just convex ; its declination is very trifling, that 

 of the shorter extremity is rather more considerable. The 

 beaks, which are acute and rather prominent, are situated 



