DREISSENA. 47 



Only one species is known in Europe; and it was first 

 noticed (in 1754) by the Russian traveller and natura- 

 list, Pallas, in the River Wolga, as well as in the Black 

 Sea. The epithets of " fluviatilis " and "marinus" 

 which he applied to these two forms, coupled with the 

 circumstance that this was antecedent to the era and 

 usage of binomial appellations, have given rise to con- 

 siderable controversy as to whether these forms belong 

 to different species or to varieties of the same species, 

 one of which has a freshwater, and the other a marine, 

 habitat. The last supposition would be quite consistent 

 with the fact observed by Nilsson, that several shells, 

 which are usually inhabitants only of fresh water, live 

 in the Baltic Sea together with other shells which are 

 peculiarly marine. 



DREISSENA POLYMOR'PHA*, Pallas. 



Mytilus polymorphus, &c., Pallas, It. Russ. i. p. 478. Dreissenapoly- 

 morpha, F. & H. ii. p. 165, pi. xlii. f. 4, 5, and (animal) pi. Q. f. 4. 



BODY dark-coloured : mantle bordered in front with greyish- 

 white, at the posterior side being yellowish or fawn-colour, 

 and striped like the shell with zigzag marks of reddish-brown; 

 cirri of the branchial orifice arranged in concentric rows, red- 

 dish-grey, with a tint of brown at their base : foot oblong and 

 cylindrical, grey, with a slight rosy hue : gills greyish : labial, 

 palps rather large, triangular, and lanceolate: byssus composed 

 of several stout and flexible threads. 



SHELL oblong, rising into a sharp keel in the middle of each 

 valve and flattened below, pointed at the end or beak, and 

 gradually, but obliquely, widening towards the front, rather 

 solid but not glossy, yellowish-brown, and often marked trans- 

 versely on the upper part with undulating or zigzag streaks of 

 purple or dark brown, strongly but irregularly wrinkled in the 

 same direction, and longitudinally but slightly puckered at 

 irregular intervals : epidermis silky ; beneath the epidermis 

 the surface is purplish-brown : beaks small, quite terminal, and 



* Many-shaped. 



