FAMILY MYTILACEA. 



209 



the centre. Individuals of this mussel are wonderfully prolific, and 

 attach themselves in groups one upon another. The shell is of 

 a peculiar triangularly trapezoid form, the valves being affixed to 

 the animal by an oblong compound adductor muscle just within 

 the posterior margin, and a small auxiliary transverse muscle 

 affixed to a cross shelf below the umbo. The hinge of the valves is 

 toothless, the cartilage by which they are connected being marginal 

 and external. 



A few species of Dreissena have been discovered in the West 

 Indies, in Central America, and in the Mississippi in the Southern 

 United States; and one at Senegal. 



1. Dreissena polymorpha. Many-shaped Dreissena. 



Shell ; triangularly trapezoid, very gibbous, obtusely keeled, ful- 

 vous, stained and trans- 

 versely undulately waved 

 with olive-brown, under 

 side broadly flatly im- 

 pressed, right valve sinu- 

 ated at the central mar- 

 gin for the passage of the 

 byssus ; interior bluish- 

 white, not'pearly. 



Mytilus polymorphus, pars, Pallas 

 (1754), Voy. Buss. App. p. 212. 



Mytilus e fluvio Wolga, Chemnitz 

 (1795), Conch. Cab. vol. xi. p. 

 256. pi. ccv. f. 2028. 



Mytilus Hagenii, Baer (1825), Inst. Solemn. Oken's Isis, part v. p. 525. 



Mytilus Volgensis, Gray (1825), Ann. Phil. p. 139; Ind. Test. Supp. p. 8. 

 pi. ii. f. 6. 



Mytilus area, Kickx (1834), Desc. nouv. esp. de Moule. 



Dreissena polymorpha, Yan Beneden (1834), Bull. Acad. Sci. Brux. vol. i. 

 p. 105 ; Ann. Sci. Nat. pi. viii. f. 1 to 11. 



Tichogonia Chemnitzii, Rossmassler (1835), Icon. vol. i. p. 113. f. 69. 



Mytilina polymorpha, Cantraine (1837), Ann. Sci. Nat. vol. vii. p. 308. 



Hob. Throughout Europe, chiefly in the northern and eastern parts. (In 

 canals, rivers, docks, water-pipes, etc.) 

 The shell of our British Dreissena, compared with that of the 



