PISIDIUM. 265 



Monog. 13. t. 20. f. 1. 3. ; Forbes and Hanleij, B. M. ii. 120. 

 t. 36. f. 1. — Cyclas obtusalis. Lam. Hist. v. 559. — Pera 

 gibba. Leach, MSS. Brit. Mus., Syn. Moll. 292. ; Dupuy, 

 Moll. Franc. 690. t. 31. f. 4. — P. Ibntinale obtusale. Held, 

 Isis, 1837,306. — P.(Cyclas) fontinalis, var. obtusalis. Dr^pwy, 

 Moll. Gers, 89. 



Young, the yellow zone broader. 



Var. 1., shell ovate, trigonal, very ventricose, blackish or 

 ocliraceous ; the edge very blunt : Cyclas obtusalis. Nilson, 

 101. — Cyclas obtusale /S. Jenyns, Monog. 13. 



Inhab. small splashy pools and other stagnant 

 waters. 



Animal white ; siphons short, obconic ; foot very 

 extensile. 



Mr. Jenyns observes : " It is by far the most active 

 and lively species that I am acquainted with, being 

 always in motion, and residing less at the bottom than 

 the rest of the family." 



121. 2. PiSiDiUM nitidum. Shining Pera. — Shell 



orbiculate, oval, very shining, finely striated ; 



umbones rather blunt, with a few deeper 



stride; cardinal and lateral teeth moderate, 



thin. (t. 12. f. 50.) 



Pisldium nitidum. Jenyns, Monog. 16. t. 20. f. 7, 8. ; Fo?-bes 

 and Hanley, B. M. ii. 126. t. 37. f. 14. — P. incertum. 

 Norm. Cycl. 6. — Cyclas nitida. Hanley, Sp. Shell, i. 90., 

 Supp. t. 14. f. 46. 



Inhab. clear water, Cambridgeshire, Battersea 

 Fields. {Gray.) 



Animal white ; siphon short, funnel-shaped, with 

 a spreading mouth, and a more or less plaited cre- 

 nated edge. 



This shell may easily be distinguished by the 

 deeper grooves on the umbo, which are more easily 

 seen in the living specimens. 



