12 UNIONID^. 



P. AMNicuM. Muller, PI. I, fig. 12. 



Ovate; very inequilateral, deeply suleated, striated; umbones, 

 somewhat blunt. 



Tellina amnica, Mull, Turt, 8fc. Cylas amnicum, 

 Turt. 



This species is more inequilateral than any other 

 Pisidium^ and considerably larger in size. These cha- 

 racteristics j, together with the deep sulcations and striae, 

 will readily distinguish it from its congeners. The , 

 valves are moderately ventricose in adult, though much 

 compressed in young specimens. It is comparatively 

 strong ; and though generally semi-transparent, it is 

 frequently scarcely diaphanous. Of an olivaceous ash 

 colour, with a broad marginal zone of a yellow tint. 



Specimens may sometimes be found nearly half an 

 inch in length, and a third in breadth. 



Hah. An abundant species ; delighting more es- 

 pecially in running streams, though by no means con- 

 fined to them. 



UNIONID^. 



Unio. Retzius. 

 Shell — Eqiiivalve, inequilateral; hinge, with two teeth 

 in each valve, the one short, irregular, and sub- 

 striated, the other elongated, compressed, lateral; 

 ligament exterior. 



Animal — Thick, 7nantle freely open in front, with simple 

 edges; branchial region fringed with numerous 

 cirrhi. Foot large, broad, compressed. 



U. TUMiDus. Retzius. PI. Ill, fig. 2. 



Wedge shaped; tumid; umbones prominent; rugose; anterior teeth 

 thick, high, and conical. 



Unio Ovalis. Flem, 8^c. 



The external appearance, both as regards colour and 

 shape, varies considerably in this shell, still it is not 

 very difficult to distinguish it from the only other re- 

 cosfnized British species. It is extremely solid, with large 

 and strong anterior teeth. The valves are unequally 

 ventricose, swelling in front, and compressed behind ; 



