306 TELLINIDiE. 



The declination of the front dorsal margin adjacent to 

 the beaks, (where it is almost straight) is very inconsider- 

 able ; the outline subsequently becomes arcuated and mo- 

 derately sloping. The anterior side is a trifle the longer ; 

 its extremity is broadly but not symmetrically rounded, 

 there being an occasional obscure subangulation at the ces- 

 sation of straightness in the dorsal edge, and the rise of 

 the lower margin exceeding the descent of the upper one. 

 The umbones are rather prominent ; the beaks acute, and 

 hardly leaning forward in the least, in front of them exists 

 (in the adult only) a linear pseudo-lunule, which is more 

 evident in the right than in the left valve. The ligament 

 is long, very prominent, and usually dark coloured. 



The ventral flexure is almost obsolete, the umbonal fold, 

 which runs adjacent to, and almost parallel with, the dorsal 

 margin, is chiefly distinguishable by the depression of the 

 surface behind it, which, in the right valve, obscurely sub- 

 angulates it. 



The hinge margin is rather broad ; there are two very 

 caducous almost rudimentary cardinal teeth in each valve 

 but no lateral lamina?. 



The length of a good sized specimen is rather more than 

 one inch, and its breadth about three-quarters of an inch. 

 A curious variety is found where fresh water mingles with 

 the sea. There is a suborbicular form taken at South- 

 ampton, with a peculiarly dull surface, and much arched 

 ventral margin ; and the declination of the hinder dorsal 

 edge, so abrupt as materially to diminish the ordinary 

 angulation of the posterior extremity. 



The animal is suborbicular, thick, and convex ; its man- 

 tle is open, and ornamented at the margin with fine white 

 filaments. The two siphons are nearly hyaline, of mode- 

 rate length, and quite plain at their orifices ; the anal one 



