UNio. 145 



anterior side, except in being" rather narrower, resembles 

 that of the preceding species ; and occupies about two- 

 sevenths (more rarely one-third) of the entire length. The 

 hinder margin in the more characteristic specimens is at 

 first retuse and then convex ; in others it is almost entirely 

 rectilinear ; in others again (yet seldom so) it is actually 

 arcuated. The ligament is elongated and projecting. The 

 umbones are not particularly prominent, and are often 

 broad. The internal nacre is silvery- white in general, but 

 occasionally stained with salmon colour : the primary teeth 

 are coarsely crenated, very much compressed, and the 

 hinder one of the left valve small or obsolete ; the lateral 

 laminse are simple, and much produced. 



A large individual measured full three inches in length, 

 and nearly an inch and a half in breadth. 



A somewhat tortuous variety is taken in the River Lea, 

 near London, and in the northern districts of England, 

 with the rostrum bending below the level of the incurved 

 ventral margin, and with the hinder side either greatly pro- 

 duced or with its upper edge arcuated. In this form the 

 primary teeth are apt to become rudimentary. 



The animal has the front margins of its mantle plain 

 and white. The branchial portion is of an orange-brown, 

 mottled with lighter and darker specks. It is of greater 

 extent than in the last species, and rather more finely 

 fringed. In a moderate-sized specimen about two hundred 

 and fifty cirrhi may be counted. The anal portion of the 

 mantle is darker, and is radiatingly striped near its mar- 

 gin with purplish-brown. The edges are quite smooth, 

 and approximate so as to form a pretty complete promi- 

 nent tube, quite distinct from the branchial region and 

 from the hinder angulated portion of the mantle. The 

 foot is very large, thick, broad, slightly apiculated, and 



V07>. II. u 



