300 CONCHIFERA. 



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, perhaps it is better 

 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 attach- 

 ment to the shell, while in Dreissena these muscular 

 cords are united into a single bundle, which has only 

 one point of attachment. In Mytilus, the branches 

 adhere through their whole length ; in Dreissena^ the 

 extremities are free, and float upon the posterior 

 transverse muscle. 



This genus was first established by Mr. Van 

 Beneden. It has also been named Tichogonia by 

 Rossmasler in 1835. He did not know the animal, 

 although it had been described eleven years before by 

 Mr. Sowerby. 



128. 1. DREISSENA polymorpha. Zebra Dreissena. 

 Shell triangular, keeled, olive, varied with black, 

 wavy-lined. 



