260 
DREISSENAD^. 
which has only one point of attachment. In 
MytiluSy 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. 
119, 1. Dkeissena polymorpha. Zebra Dreissena. 
— Shell triangular, keeled, olive varied with 
black, wavy-lined. 
Fig * 63. 
Mytilus Yolgae. Chemn* xi. 205. f. 2028. — Mytilus poly- 
morphus. Pallas, Voy. Russ. App. 211.; Sow. Gen. Shelly 
f. 4. — Mytilus Chemnitzii. Ferussac. — Mytilus lineatus. 
Waardenhurg^ Mol. Belg. — - Mytilus area. Kickx., Monog. 
— Mytilus ? volgensis. Gray, Anii. Phil. 1825, in Wood, 
Supp. t. 2. f. 6. — Mytilus Hagenii. Baer, Fer. Bull. Sci. 
iVa^. 1826, 140. — Mytilus Tore ji. /S'/ewz. — Mytilina poly- 
