334 DONACID^. 



previously mentioned. The extremity of the posterior side, 

 which is ahout half as long as the other, is very bluntly 

 wedge-shaped ; its dorsal area is more or less slightly flat- 

 tened (with the lips, however, projecting), and excepting 

 near the acute and slightly inclined beaks, traversed by 

 more or less close concentric strise, and occasionally even 

 by some minute crowded radiating striw near the ligament. 

 The umbonal ridge is well developed, but obtuse. The 

 inner edge is finely crenated posteriori}^ and very strongly 

 so at the ventral portion of it ; anteriorly it is simple. 

 The central triangular primary tooth of the right valve is 

 profoundly bifid, or even bicuspidate, the broadly diverging 

 narrow ones of the opposite valve are simple ; there are 

 two small approximate lateral teeth in the left valve, of 

 which the anterior is nearly rudimentary, and almost 

 adjacent to the primary. 



The average size of specimens is about an inch and a 

 fifth in length, and about eleven-sixteenths of an inch in 

 breadth ; the Stornaway variety exceeds an inch and a 

 half in length, and is of proportionate width likewise. 



The fry of this species is certainly the D. ruher of Tur- 

 ton's Dithyra, as we have ascertained from his cabinet ; 

 possibly also that of Montagu (Test. Brit. Suppl. p. 88. — 

 Turt. Conch. Diet. p. 4-3), but the fact is not equally well 

 assured. Young shells are almost destitute of any stria- 

 tion ; they retain, however, their characteristic form, and 

 are peculiarly and tolerably evenly compressed. 



The animal is oblong, and rather thick. Its mantle is 

 freely open in front, and has the margins fimbriated. Ac- 

 cording to Mr. Clark, the edges are double, the outer one 

 plain or slightly crenulated, the inner furnished with a 

 close set fine white fringe every where bordering it, except 

 at the ligament, and composed of alternately longer and 



