CLASS II. 

 CONCHIFERA; OR BIVALVES. 



Animals soft, without articulations, destitute of a head or 

 organs of vision, and always fixed within a bivalve shell ; 

 provided with external branchiae, a simple circulation, and an 

 unilocular heart. 



All the species are aquatic, living either in the sea or fresh 

 water. None of the animals have an internal shell. The body 

 is invariably soft, and the mouth is situated near the left side of 

 the hinge. 



ORDER MONOMYARIA. 



Animals provided with but one muscle of attachment, or ad- 

 ductor muscle, which leaves one subcentral muscular impression 

 inside of each valve. 



Family I.— MYTILACEA. 



Hinge with a linear, subinterior, marginal ligament, very 

 entire, and occupies a considerable portion of the anterior mar- 

 gin ; shells generally foliaceous. 



Genus I.— DREISSENA— Van Beneden. 



Shell boat-shaped, or mytilform ; valves carinated ; ligament 

 internal, except in a httle fissure in both valves externally ; a 

 rude cardinal tooth is situate under the umbo in the right 

 valve, which locks into a corresponding cavity in the left valve ; 

 in the umbonal angle of both valves is placed a transverse par- 

 tition, for sustaining a closing muscle ; anterior marginal scam 

 with a fissure near its centre, for the passage of the byssus. 



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