British species of Cyclas and Pisidium. 301 



Gen. II. PISIDIUM. 



* Testa parim incequilaterali . 

 Sp. 1. P. obtusale, Pfeiffer.— Tab. xx. Fig. 1—3. 



P. testa globosa, oblique subovali, tenuissime striata; umbonibus pro- 

 minulis, obtusissimis. 



Long. If lin. Alt. l^lin. Crass, vix. 1^ lin. 



Pisidium obtusale, Pfeiffer, 125. t. 5. f 21, 22. — Brawn in Edinb. 

 Journ. of Nat. and Geog. Scien. 1. 413. 



An Cyclas obtusalis ? Lamarck, 5. 559- 



Pera gibba, Leach, MSS. Brit. Mus. 



Var. /3. 



Testa ovato-trigona, ventricosissima, margine obtusissimo. 



Cyclas obtusalis, Nihson, 101. 



Animal album ; tubo siphonali abbreviato, subconico ; pede valde ex- 

 tensili, testa dimidio et ultra longiori. 



Testa globoso-ovalis, ventricosissima, crassitudine fere altitudinem aequanti, 

 nitida, subtiliter striata ; plerumque virescenti-nigra vel ochraceo- 

 nigricans, zona marginali (junioribus latissima) lutescenti, interdum 

 subaurantia. ; rarius omnino lutescens : umbones tumidi, obtuse ro- 

 tundati, paulo prominentes. 



Var. /3. gaudet testa ventricosiori, margine basali obtusissimo, quo minua- 

 tur altitudo, et forma magis trigona vel ovato-trigona provenit. 

 Hsec varietas plerumque nigricans, ocbraceo plus minus ve fucata. 



This species, which is distinguished from all its congeners by the 

 extreme convexity of the shell, is certainly the P. obtusale of Pfeiffer. 

 and probably the Cyclas obtusalis of Lamarck, but from the short de- 



