DREISSENIR/E. 521 



Fam. DKEISSENIDjE. 



Animal oblong. Mantle closed, except a passage for the 

 foot and the two siphonal orifices ; branchial opening pro- 

 longed into a tube with a circular, fringed orifice ; anal 

 opening plain and sub-sessile. Foot short, ligulate, with a 

 conspicuous byssal groove. 



Living in fresh water ; byssiferous. 



In the Dreissenidai the microscopic structure of the 

 shell, as shown by Dr. Carpenter, is very different from 

 that of the Mytilidce, the internal layer being composed of 

 large prismatic cells, with a layer of brownish colour under 

 the epidermis which also shows traces of a cellular struc- 

 ture. 



Genus DREISSENA, Van Beneden. 



Shell equivalve, very inequilateral, sub-triangular, tumid ; 

 surface of valves covered with an epidermis ; beaks terminal, 

 furnished internally with a transverse shelf or septum. 

 Hinge composed of an imperfectly-developed cardinal tooth 

 in the right valve, with a corresponding socket in the left ; 

 ligament linear, internal. Three muscular impressions; 

 pallial impression obscure. 



Syn. Mytilina, Cantr. Tichogonia, Bossm. Cselo- 

 gonia, Bronn. Dithalmia, Say. Enocephalus, Mtinst. 

 (foss.) Congeria, Partsch. (foss.) Mytilina, Mytilomya, 

 Cantr. (foss.) '? Myoconcha, D'Orb. 



Ex. D. polymorpha, Pallas, pi. 122, fig. 2. Shell, D. 

 polymorpha, fig. 2, a, 2, b. 



The type of this genus, Mylilus polymorphus of Pallas, 



