22 CO^CEYLl A— DITHYRA. 4. 



Shell nearly an inch long, and an inch and a half broad, white 

 covered with a yellowish brown skin, regularly striate transversely, 

 with one of the valves generally much more tumid than the other : 

 beaks nearer the middle than in the last species. 



This species is subject to much variation of figure, being fre- 

 quently nearly globular, but always rounded and somewhat open at 

 the ends. Sometimes it is found attached to clefts in the rocks, 

 but not embedded in them, when there is a considerable opening 

 in the front margin for the issue of the byssus of attachment. 



It was observed in great abundance, in Melville Island, in the 

 Polar regions, three miles distant from the shore, where it had pro- 

 bably been cast by the agitation of the sea, and when deprived of 

 the inhabitant driven inwards by the violence of the winds, by 

 Mr. Griffiths, an officer in Capt. Parry's late northern expedition. 



The specimens we examined, in no respect differed from our native 

 ones, being regularly striate transversely, with both the extremities 

 rounded and a little gaping : but they were larger. 



plicata. Saxicava testd ovatd, transversim striata, extremitatibus rotundatis 

 ^' subhiantibus. 



Shell oblong, dilated and open at the anterior end, and transversely 



laminar. 

 Mytilus plicatus. Chemnitz, viii. p. 153, tab. 82, fig. 733. 

 Gmelin, Syst. p. 3358. 

 Pennant, Brit. Zool. iv. p. 242 

 Turton, Linn. Syst. iv. p. 295. 

 Montagu, Test. Brit. Suppl. p. 70. 

 Dillwyn, Descript. Catal. p. 306. 

 Turton, Conch. Diet. p. 114. 



