268 PISIDIAD^. 



tlien gradually rise to the umbo as the shell in- 

 creases in size by the addition of new laminas 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, vili. 60. ; 

 Dillwyn, D. Cat. i. 105. ; Turt. Diet. 1G8. — Tellina striata. 

 Schruet. Flussconch. 193. — Tellina rivalis. 3Ia(on, Linn. 

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

 Cordula amnica. Leach, Syn. Mull. 293. — Cardium amni- 

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

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

 Nilso)!, 99. — Pisidium obliquum. Pfeiffer, 124. t. 5. f. 19, 

 20. — Cyclas amnica. Fleming, B. A. Ao'i.\ 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. 11—15.; Forbes and Ha?iley, B. M. ii. 133. t. 37. 

 f. 8, 9., t. O. f. 8.— Cyclas Pfeifferi. Ziegl. — ViskVmm 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 Bi-it. Mils., Syn. Moll. 292. — Pisidium amnicum /3. 

 Jenyns, Mon. 22. 



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

 sloiviana. 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. 



