110 NAYADES. 



umbones, there are indications of remote, nearly obsolete 

 stria) ; internal surface of a bluish-white, with iridescent reflec- 

 tions. Length not a third of its breadth ; thickness four-fifths 

 of its length. 



Figs. 1 and 2, pi. XXI, external and internal representation 

 of the teeth. 



This shell tapers more abruptly in thinness towards the pos- 

 terior side, than any of its British congeners ; and is consider- 

 ably more obtuse at its anterior side, than the U. pictorum ; but 

 its much more elongated form, will at once distinguish it from 

 that shell. 



I found this species in the canal, near Hull ; and William 

 Thompson, Esq., of Belfast, procured it in the Avon, near 

 Leamington. 



4. Unio TUMiDus, pi. XXI, f. 8, 9. 



Unio tumidus, Retzius, NoVa Testaceor. Genera, p. 17, No. 

 3 ; Unio tumida, PfeifFer, II, p. 34, pi. 7, f- 2, 3, and pi. 8, f. 

 1, 2 ; Rossmassler, II, p. 117, pi. 3, f. 70 b; lb.. Ill, p. 27, pi. 

 3, f. 202, 203; Alder, Mag. Zool. and Bot., II, p. 118; Mi/sca 

 solida, Turton, Man., p. 22, pi. 2, f. 13; lb.. Conch. Diet., p. 

 246, pi. 16, f . 2 ; Unio tumidus, Brown, lUust. Conch., p. 82, 

 pi. 32, f. 5, 6, 7, 8. 



Shell very thick, solid, cuneiform, transversely oblong-ovate, 

 much inflated; umbones produced, and placed very close, situate 

 near one extremity; anterior side short, rounded; posterior side 

 long, graduafly sloping from the umbones both above and below, 

 and terminating in a subtruncated beak ; cardinal tooth large, 

 single, thick, elevated, and finely serrated at the edge, in the 

 right valve, and double in the opposite valve; muscular im- 

 pressions small, and deeply sunk both before and behind ; pallial 

 impression strongly defined; external surface with rather strong, 

 concentric wrinkles, and the whole of a reddish-brown, slightly 

 tinged with olivaceous-green ; inside white, with but little iri- 

 descent lustre. Length half its breadth ; thickness four-fifths 

 of its length. 



Figs. 5 and 6, pi. XXI, external and internal representation 

 of the teeth. 



Found in the New River, near London ; and the Avon, near 

 Leamington, Warwickshire. 



