DREISSENA. 



Fig. 62. 



259 



Group of Dreissena polymorpha, on an old shell, 

 a, Lower siphon ; a*, magnified : b, Upper siphon ; c, Byssus. 



Mytilus as the Iridince do to the Uniones, as they both 

 differ from their apparently allied genera by the ad- 

 hesion of the lobes of the mantle, and the possession 

 of a siphon ; but in our present imperfect state of 

 knowledge of the animals of the Conchifera, and of 

 the value of the adhesion or the separation of these 

 parts in a systematic point of view, it may be bet- 

 ter to regard them as the type of a distinct family. 



The fossil genus Congeria agrees with Dreissena 

 in many particulars, and perhaps belongs to the same 

 family, if it is in reality a separate genus. 



1. DREISSENA Van Beneden. (Dreissena.) 



The animal of this genus differs from Mytilus in the 

 mantle being closed, while in the latter it is open. 

 In the latter, the retracting muscles are divided 

 into several bundles, each of which has its proper 

 attachment to the shell, while in Dreissena these 

 muscular cords are united into a single bundle, 



8 2 



