12 UNIONID^E. 



P. AMNICUM. Mutter. PI. I, fig. 12. 



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

 somewhat blunt. 



Tellina amnica, Mull, Turt, Sfc. Cylas amnicum, 

 Turt. 



This species is more inequilateral than any other 

 Pisidium, and considerably larger in size. These cha- 

 racteristics, together with the deep sulcations and strise, 

 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. 



Hob. An abundant species; delighting more es- 

 pecially in running streams, though by no means con- 

 fined to them. 



UNIONISE. 

 UNIO. Retzius. 



Shell Equivalve, inequilateral; hinge, with two teeth 

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

 striated, the other elongated, compressed, lateral; 

 ligament exterior. 



Animal Thick, mantle 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, fyc. 



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- 

 cognized British species. It is extremely solid, with large 

 and strong anterior teeth. The valves are unequally 

 ventricose, swelling in front, and compressed behind ; 



