PISIDIUM* 25 



by a local name, when we have at least equally good 

 reasons and authority for adopting a name bestowed by 

 an earlier writer, and which, besides, is not liable to the 

 same objection. This species differs from P. fontinah 

 in its shape being oval instead of triangular, and in its 

 beaks being more central, and blunt or compressed. Its 

 colour is also yellowish- white instead of grey ; and it has 

 much less lustre. It is the Cyclas fontinalis of Nilsson. 



C Round. 



4. P. ni'tidum*, Jenyns. 



P. nitidum, Jen. p. 16, tab. xx. f. 7, 8 ; F. &H. ii. p. 126, pi. xxxvii. f. 14. 



Body whitish, -^dth sometimes a faint tinge of yellow, caused 

 by the colour of the liver : tube short, funnel-shaped ; orifice 

 wdde, and its edges notched or puckered : foot rather long, 

 thin, slender, and finely pointed : mantle bordered with grey. 



Shell suborbicular, compressed except in the upper part 

 where it is rather ventricose, thin, extremely glossy, iridescent 

 (especially in the young state and near the beaks) , finely and 

 regularly striated or ribbed concentrically, with from 3 to 5 

 separate and deeper grooves which encircle the umbonal region, 

 the strise or ribs being rather broad ; yellowish- white or light 

 horn-colour : epidermis a mere film : anterior side somewhat 

 truncate, but rounded: posterior side slightly produced and 

 sloping abruptly below : lower margin rounded : healcs nearly 

 central, rather prominent, but obtuse : ligament very short, 

 and scarcely discernible : inside w^hitish, and plainly showing 

 the scars of the adductor muscles and mantle : hinge and teeth 

 as in the two last species. L. 0*15. B. 0*15. 



Var. splendens. Shell of a lemon-colour, nearly half as 

 large again as that of the ordinary form, stronger, less glossy, 

 rather more obhque, and less deeply striated, with the beaks 

 more swollen and the hgament stronger and perceptible. 



Habitat : Lakes, ponds, and standing water in all 

 parts of the kingdom from Zetland to the Channel 

 Isles. Malm has described and figured it as a Swedish 



* Glossy. 



C 



