BKEISSENA. 
259 
Fig. 62. 
Group of Dreissena polymorpha, on an old shell. 
a, Lower siphon ; a*, magnified: b, Upper siphon ; c, Byssus. 
Mytilus as the IridincB 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. Dkeissena 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, 
