98 MYTILACEA. 



1. Dreissena polymorpha, pi. X, f. 7, 8, 9, 10, 11, 12. 



Dreissena polymorplm, Van Beneden, Ann. Sci. Nat., 1835, 

 p. 210, pi. 8, f. 1 to 11 ; Strickland, Mag. Nat. Hist., 1838, p. 

 361; Alder, Mag. Zool. and Bot., II, p. 117; Tichogonia 

 Chemnitzii, Rossmassler, I, p. 113, pi. 3, f. 69; Mytilus poly- 

 morphus, Pallus, Voy. Russ. App., p. 211; Mjjlilus Wolgce, 

 Chemnitz, XI, p. 205, f. 2028 ; Mytilus Hagenii, Baer, Per. 



Bull. Sci. Nat., 1826, p. 140; ? Volgensis, Gray, Ann. 



Phil., 1825; Dreissena polymorpha, Brown, lUust. Conch., p. 

 76, pi. 29, f. 4, 5, 6, 7, 8, 9. 



Shell cymhiform, extremely flat in front, and acutely carinated 

 behind; umbones incurved, pointing anteriorly, turned down- 

 wards at the points, and very closely pressed together; anterior 

 side with an elongated, cordiform groove, reaching nearly to 

 the centre of the valves, towards the base of which is a large, 

 wide, elongated hiatus, acute at both ends, always situate in 

 the right valve, for the passage of the byssus ; the edge of the 

 opposite valve being always entire ; external surface covered 

 with a rather strong, olivaceous, irregularly wrinkled epidermis, 

 with indications of nearly obsolete, longitudinal, divergent ribs, 

 or elevations; beneath this the shell is beautifully mottled with 

 zigzag brown, or purple, transverse clouds or markings ; inside 

 dull white, with the pallial and muscular impressions of a purple 

 hue, and very glossy. 



This species is subject to great variety in length and breadth, 

 varying from an inch and five-eighths to an inch in length. 

 Some are elongated and narrow ; others short and broad. It 

 inhabits the commercial docks in various parts of Great Britain, 

 and is to be met with in most of our canals. It is common in 

 the Bridgewater canal, and also in many places in the canal 

 betwixt Manchester and Hull ; and was found by my friend 

 Mr. Stark in the Union canal, near Edinburgh, in 1834. The 

 animals are gregarious, generally found adhering in clusters by 

 a strong byssus. 



This shell is not an aboriginal of Britain, being a native of 

 the Volga, the Danube, and other continental rivers, and has 

 been introduced upon timber ; but is now so generally diffused 

 throughout Britain, that it may fairly be considered as a British 

 shell. ^ 



