260 



DREISSENADJE. 



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 

 Kossmasler in 1835. He did not know the animal, 

 although it had been described eleven years before 

 by Mr. Sowerby. 



119. 1. DREISSENA polymorpha. Zebra Dreissena. 

 Shell triangular, keeled, olive varied with 

 black, wavy-lined. 



Fig. 63. 



Mytilus Volga?. Chemn. xi. 205. f. 2028. Mytilus poly- 

 morphus. Pallas, Voy. Russ. App. 211.; Sow. Gen. Shell, 

 f. 4. Mytilus Chemnitzii. Ferussac. Mytilus lineatus. 

 Waarderiburg, Mol. Belg. Mytilus area. Kickx, Monog. 

 Mytilus? volgensis. Gray, Ann. Phil. 1825, in Wood, 

 Supp. t. 2. f. 6. Mytilus Hagenii. Baer, Per. Bull Sci. 

 Nat. 1826, 140. Mytilus Toreyi. Stenz. Mytilina poly- 



