10 SPH^RIID^. 



name given by Draparnaud to some one of tlie nume- 

 rous varieties of either Miiller^s species or S. corneum. 

 The Cyclas rhomboidea of Say_, to which Dr. Gray sup- 

 posed our shell to be allied, is only a fourth of an inch 

 long, and, according to Gould, is an obscure or doubtful 

 species. Its nearest congener in this country appears 

 to be S. rivicola ; but it may be readily distinguished 

 from that species by its oblong and subangular shape, 

 thinner texture, much paler colour and fainter striae, 

 and especially by its straight hinge-line. The Devon- 

 shire and Lancashire sj^ecimens are of a darker colour 

 than those from the Paddington Canal. The young 

 exhibit the same form as the adult ; and, like the other 

 species, their shells are slightly iridescent. 



4. S. lacus'tre^, Miiller. 



Tellina lacustris, Miill. Verm. Hist. pt. ii. p. 204. Cyclas caliculata, 

 F. & H. ii. p. 115, pi. xxxvii. f. 7 (as C. lacustris), and (animal) pi. O. 

 f.7. 



Body whitish, slightly tinged with grey or rose-colour : 

 tuhes long ; the branchial one cylindrical and truncate at its 

 orifice, which is large ; the other rather conical, and having 

 a smaller opening : foot nearly twice the length of the shell, 

 obtuse at its extremity : mantle fringed with grey. 



Shell nearly round, or subrhombic, equilateral, compressed, 

 especially towards the lower and side margins, extremely thin, 

 glossy and semitransparent, light horn-colour, or greyish, 

 with sometimes a few darker zones and an iridescent hue, 

 very faintly striated concentrically : epidermis very thin : an- 

 terior and posterior sides cut ofi' and sloping from shoulders on 

 the upper or dorsal side towards the front margin, which is 

 slightly curved and has sharp edges: heahs central, very pro- 

 minent, and capped with the fry or nucleus of the shell, Avhich 

 Ls more globular than in the subsequent stages of growth : 

 ligament narrow, thin, and just discernible on the outside : 

 inside bluish-white, with very little nacre, owing to the thin 

 texture of the shell : hinge rather strong ; teeth arranged as 



* Inliabiting lakes. 



