68 CONCHOLOGY. 



distant lateral teeth upon the left valve, corresponding with two 

 precardinal lamellous teeth, and one distant lateral tooth upon 

 the right valve ; ligament anterior, elongated ; a large, bifid ; sub- 

 median muscular impression ; another anterior one smaller and 

 less distinct. Inhabits the Indian ocean. Six living species. 

 One fossil species. 



Tridacna gigas. Tridacna crocea. 



T. squamosa. T. mutica. 



T. elongata. T. serrifera. 



2. Genus Hlppopus. 



Animal. As above. 



Shell. More elongated and inequilateral than in the Tridacna, 

 the anterior side being longer than the posterior ; the posterior 

 slope closed with a dentated margin. Inhabits the Indian seas. 

 One species. 



Hippopus Tnaculatus. 



FAMILY XVI. 



MxTiLACEA. Three genera. 



1. Genus JMytihis. 



Animal. Body oval, somewhat inflated ; mantle open in its 

 inferior half only, and terminated posteriorly by an oval cleft with 

 fringed edges ; a linguiform, canaliculated, abdominal appendage, 

 with a byssus at its base behind, and several pairs of retractile 

 muscles ; mouth with simple lips ; two contractile muscles, of which 

 the anterior is very small. 



Shell. Of a serrated tissue, elongated more or less oval, some- 

 times subtriangular, equivalve, summits anterior, more or less 

 curved, sloping inferiorly in a slight degree ; hinge toothless, or 

 with two very small rudiments ; ligament dorsal, linear, subinte- 

 rior inserted in a narrow and very long furrow ; two muscular 

 impressions, of which the anterior is very small, besides those of 

 the retractile muscles. Inhabits the British seas. Thirty-six 

 species. 



Mytilus erosus. Mytilus Magellicanus. 



