26 Lamarck's Genera of Shells. 



dinal lamina, truncated or forming a sinus at its anterior exter- 

 mity. Two distant muscular impressions, lateral, subgeminal. 

 Ligament linear, external, its anterior extremity inserted in the 

 sinus of the cardinal lamina. 



The anodontae, which Linnseus confounded with the Mytili, 

 are fresh water shells, usually very thin, and often of a large 

 size. They greatly resemble the Uniones, but have neither 

 cardinal nor lateral tooth, the hinge presenting merely a smooth 

 interior margin, or lamina, situated immediately below the 

 nymphae, and terminated anteriorly by a truncation or sinus. 

 The shell is nacreous, and covered externally with a thin, 

 greenish, false epidermis; beaks decorticate, oblique, partly 

 inclining to the posterior margin. The animal has two short 

 tubular apertures, formed by the posterior extremity of the 

 mantle, and furnished with little tentacular threads. It has 

 nobyssus; it has a very large, almost round, compressed muscu- 

 lar foot, which it uses for locomotion. It is hermaphrodite, and 

 seems to be viviparous, for the ova pass between the branchiae, 

 ■where the young are found with their shell perfectly formed. 



The species are subdivided into(l) shells without any distinct 

 angle at the posterior extremity of the cardinal line, 10 species ; 

 andC2) those which are distinctly angular at that part, 5 species. 

 Type. Anodonta Cygneus*. (Mytilus cygneus, Linn.) 



Shell ovate, brittle, posteriorly dilated, rounded ; with un- 

 equal transverse furrows ; nates obtuse. Lakes, §c of Europe. 

 In all 15 species. PI, I. Fig. 71. 



4. Iridinat. 



Shell equivalve, inequilateral, transverse ; beaks small, slight- 

 ly curved, almost straight. Muscular impressions as in the 

 Anodonta. Hinge long, linear, attenuated towards the middle, 

 tubercular through its whole extent, almost crenate ; tubercles 

 unequal, frequent. Ligament external, marginal. 



The principal difference between the Anodonta and Iridina, 

 consists in the tuberculated hinge of the latter, in other re- 



* From eygnus, a suan. t From iris, a rainbow. 



