in Northamptonshire, 



573 



field and myself, on the piers of the bridge which crosses the 

 Nen at Fotheringhay ; and again, a little higher up the same 

 river, on the stones of a small overfall at Tansor. I have 

 every reason to believe that its establishment in the Nen is of 

 recent date (subsequent to 1828), and that it was introduced 

 from Wisbeach on timber. I have formerly, with my friend 

 Mr. Lowe, examined the stream carefully for several succes- 

 sive years, both above and below the spots where it occurred, 

 but never detected a single specimen. 



No species of the 

 order Mollusca ap- 

 pears to be so indif- 

 ferent to locality, as 

 it is now found in 

 almost every part of 

 Europe, in inland 

 seas, marshes, canals, 

 tanks, and running 

 streams. A new ge- 

 nus, whose characters 

 are very distinct from 

 those of ikfytilus, hav- 

 ing been proposed for 

 this species by Dr. 

 Vanbeneden, whose observations are given in the Annates des 

 Sciences Naturettes (vol. iii. p. 193. n.s.), it may not be un- 

 interesting to such conchologists as have not access Jo the 

 paper to copy its characters : 



Dreisse x n^ * Vanbeneden. (fig. 73.) 



Animal. Mantle entirely closed, with the exception of 

 three apertures, one of which is furnished with a siphon ; an- 

 terior extremity of the body bifurcate, and including between 

 the segments the anterior transverse muscle (which is at- 

 tached to the septa) ; abdomen depressed ; extremities of the 

 branchiae free through their posterior half. 



Shell regular, equivalve, inequilateral ; beaks terminal, fur- 

 nished within with a septum ; muscular impressions three, of 

 which the central is simple and linear. 



The principal differences between this genus and Mytilus, 

 as indicated by the generic character, are as follows : — In 

 Mytilus the mantle is open; in Dreissena closed. In the 

 former the retracting muscle is divided into several bundles, 

 each of which has its proper attachment to the shell; while 



* So named after M. Dreissens, its first discoverer in Belgium. 



