46 DREISSENID^. 



Mytilid(2 ; but the mantle of the animal in the present 

 family is nearly closed^ and the hinge of the shell is 

 furnished inside with a transverse plate or septum, which 

 is a peculiar and unmistakeable feature. Mytilus has 

 also several cardinal teeth; but I have failed to detect 

 any in the British species of Dreissena, although the 

 authors of the ' British Mollusca ' and Moquin-Tandon 

 mention an " obscure apical elevation/' or "dent cardinale 

 a peine saillante/' in the right valve of this species. 

 However, cardinal teeth certainly do exist in three species 

 of Dreissena from the Indian and Pacific Oceans, which 

 have been described by Erichson in Wiegmann's ' Archiv 

 fiir Naturgeschichte ' for 1836. The same author also 

 incidentally remarks that the European species (which 

 he called Tichogonia fluviatilis) has a rudimentary tooth. 

 The mantle is for the most part closed and produced 

 into tubes on the posterior side, instead of being open 

 throughout as is the case in Mytilus. 



Genus DBEISSE'NA *, Van Beneden. PI. II. f. 3, 4, 5. 



Although the establishment of this genus is due to 

 Van Beneden, the probability of its separation from 

 Mytilus was first suggested by an equally distinguished 

 zoologist of our own country. Dr. Gray. The Dreissence 

 may be called " freshwater Mussels " with greater pro- 

 priety than the Uniones, or Anodonta ; and the habit and 

 faculty which the Dreissenoi possess, in common with the 

 true Mussels, of mooring or attaching themselves by a 

 strong byssus to extraneous substances, put us very 

 much in mind of their marine analogues. They are also 

 equally gregarious and capable of living for a long time 

 out of water. 



* Named after M. Drcissens, a druggist at Mazeylh. 



