140 CLASS I. TROPIOPODA. ORDER II. UN1MUSCULOSA. 



his ' Voyage en Russie,' is characterized as having a small septum, 

 forming a cavity just beneath the umbo in each valve. The genus Dreis- 

 sena is founded upon this feature, together with a slight modification of 

 the animal ; a similar indication may, however, be traced in some of the 

 Pinna and other Mytilacea. 



The shell of Mytilus is described as being equivalve, oblique, and 

 somewhat triangular or wedge-shaped ; the umbones are acute, nearly 

 straight, and terminal ; and the shell is always rounded at the posterior 

 side. There are no teeth, but in some species the hinge margin is a little 

 crenulated, exhibiting an indication either of teeth, or of the crenulated 

 ligamental pits which characterize the hinge of the Aviculacea. The 

 ligament is marginal and partly internal. The muscular impression is 

 compound and sublateral. 



Examples. 



PL CII. Fig. 1. 



Mytilus achatinus, Lamarck, Anim. sans vert., new edit., vol. vii. 



p. 45. Enc. Meth., pi. 218. f. 3. 

 Mytilus latus, Chemnitz. 

 Var. Mytilus versicolor, Gmelin. 

 Mytilus variegatus, Chemnitz. 



PL CII. Fig. 2. 



Mytilus crenatus, Lamarck, Anim. sans vert., vol. vii. p. 38. Enc. 

 Meth., pi. 217. f.3. 



PL CII. Fig. 3. 



Mytilus polymorphus, Pallas, Voy. en Russie, App., p. 211. Gmelin, 



p. 3363. 

 Mytilus (e fluvio Volga) , Chemnitz. 

 Dreissena polymorpha, Vanbeneden. Gray. 



