268 



then gradually rise to the umbo as the shell in- 

 creases in size by the addition of new laminae of 

 shelly matter to its edge. 



The apex of the umbo is sometimes deficient. 



125. 6. PISIDIUM amnicum. Shell ovate, ventricose, 



deeply sulcately striated; umbones rather 

 blunt, simple ; cardinal teeth prominent, well 

 developed, diverging like a V ; lateral teeth 

 moderate, thin. (t. 1. f. 5.) 



Tellina amnica. Miiller, ii. 205. ; Linn. Trans, viii. 60. ; 

 Dillwyn, D. Cat. i. 105. ; Turt. Diet. 168. Tellina striata. 

 Schroct. Flussconch. 193. Tellina rivalis. Malon, Linn. 

 Trans, ii. 44. t. 13. f. 37, 38.; Don. B. Z. t. 64. f. 2. 

 Cordula amnica. Leach, Syn. Moll. 293. Cardium amni- 

 cum. Montag. T. B. 86. Cyclas palustris. Drap. 131. 

 t. 10. f. 15, 16. Cyelas obliqua. Lam. Hist. v. 559.; 

 Nilson, 99. Pisidium obliquum. Pfeiffer, 124. t. 5. f. 19, 

 20. Cyclas amnica. Fleming, B. A. 453. ; Turt. Biv. 250. 

 t. 11. f. 25., Man. ed. 1. 15. f. 5. Pisidium amnicum. 

 Jenyns, Mon. 21. t. 21. f. 2.; Moq. Tand. M. F. ii. 533. 

 t. 52. f. 1115. ; Forbes and Hanley, B. M. ii. 133. t. 37. 

 f. 8, 9., t. O. f. 8. Cycias Pfeifferi. Ziegl. Pisidium inter- 

 medium. Gass. Pisid. Aquit. 11. t. 1. f. 4. P. Gratelou- 

 peanum. Norm. Cycl. 4. P. inflatum. Megerle. 



Young shell rather compressed ; umbones scarcely prominent. 



Var. 1., shell with very deep grooves. Pera fluviatilis. Leach, 

 MSS. in Brit. Mvs., Syn. Moll. 292. Pisidium amnicum /3. 

 Jenyns, Mon. 22. 



Var. 2., shell nearly smooth, with slighter striae. Pera Hen- 

 slowiana. Leach, MSS. Brit. Mus. Pisidium amnicum y. 

 Jenyns, Mon. 22. 



Inhab. rivers and gently running streams, residing 

 wholly at the bottom, and partly buried in the mud. 



Animal white ; siphon rather variable, obliquely 

 truncated. 



126. 7. PISIDIUM cinereum. Shell greyish or cine- 



reous, rather compressed, oval, finely striated, 



