CH. Vl] THE FRESH-WATER MUSSEL 203 



U. tumidus (Retz), 3" x l", oval, umbones with angular wrinkles, 



confluent and concentric, lower margin curved. 

 genus Anodonta, hinge-teeth rudimentary or absent, lunule in- 



distinct, shell thin. 



A. cygnea (L.), 5^"x2", oblong, umbo at one-fourth from 

 anterior end ; muscular impressions indistinct. A. anatina 

 is but a variety of A. cyynea 1 . 



Family SPH^RIID^. Shell suborbicular and thin ; hinge with 



cardinal and lateral teeth; ligament external and placed on 



posterior side of hinge ; foot large and tongue-shaped, no 



byssus ; one siphon, or two siphons more or less united. 



genus Sphceriuin Scop., shell nearly equilateral, umbo near 



middle of dorsal margin ; two siphons. 

 S. corneum (L.), f'x^", yellow horn-colour with faint con- 



centric bands. In June, 1901, masses of this species blocked 



the inflow of the bathing tank at Christ's College, Cam- 



bridge. 

 S. lacustre (Mull.), f'x^y", yellowish-white or ashy -grey, thin, 



compressed, sides truncated and sloping from dorsal 



margin. 

 S. pallidum Gray, f'xf", drab or yellowish, anterior side 



rounded, posterior side truncated. 

 S. rivicola (Leach), fo" x T V, horn-coloured with dark concentric 



bands and deep ridges, ligament very conspicuous. 

 genus Pisidium (Pfeiffer), shell inequilateral, one siphon. 



P. amnicum (Miill.), f"x^", shell triangular, inequilateral, with 



the anterior side the longer. 

 P. subtunicatum Malm, \" x ^", shell triangular, striated con- 



centrically, posterior side much produced and rounded ; 



var. henslowana has plate-like appendage to umbo. 

 P. nitidum Jenyns, \" x }", shell round, anterior side truncate 



and rounded, posterior side produced and sloping abruptly. 

 P. pusillum (Gmelin), \" x ", shell oval, both sides rounded 

 and compressed. 



1 Woodward, "List of British Non-Marine Mollusca," Journal of 

 Conchology, October 1903, p. 352. 



