CONCHIFERA. 



297 



geria Partsch ; Mytilina Cant.) — Shell equivalve, very 

 inequilateral, subtriangular, tumid, surface 

 covered with an epidermis ; beaks terminal, 

 furnished internally with a transverse shelf or 

 partition; hinge composed of an imperfectly 

 developed cardinal tooth in the right valve, 

 and a corresponding socket in the left; liga- 

 ment linear, internal ; three muscular impres- 

 sions, pallial impression obscure. Animal, 

 oblong, its mantle closed, excepting a passage Dreyssina poly . 

 for the foot, and the two siphonal orifices ; bran- wlOT ^ MS - 

 chial opening prolonged into a tube, with a circular fringed 

 orifice ; anal opening plain, and subsessile ; foot short, 

 ligulate, with a conspicuous byssal groove.* — About 6 

 species ; also fossil. 



This shell is found in the rivers of Europe, also in the 

 Caspian and Black Seas, and has become abundant also in 

 this country, probably brought from some of the above- 

 mentioned places, adhering by its byssus to the bottoms of 

 vessels. It was first found in the Commercial Docks in 

 1824. They are often found attached to each other in 

 great numbers, either in fresh or brackish water. The 

 'peculiar triangular form, the dark zigzag markings upon a 

 light ground, and yellowish epidermis, make these shells 

 easily distinguished. They were formerly called Mytilus 

 polymorphic. 



Mytilimeria. Conrad. — Shell equivalve, tolerably 

 oval, thin, the bosses somewhat rolled inwards; the 

 hinge is toothless, with a shallow hole under the bosses ; 

 the pallial impression with a broad blunt sinus. Animal, 

 unknown. — 1 species. 



Found on the coast of California, attached to sea- weed 

 roots. 



* Forbes's British Moll. 



