10 



name given by Draparnaud to some one of the nume- 

 rous varieties of either Muller'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 strias, 

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

 shire and Lancashire specimens 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, Mull. Verm. Hist. pt. ii. p. 204. Cyclas caliculata, 

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



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

 tubes 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, com- 

 pressed, especially towards the lower and side margins, ex- 

 tremely thin, glossy and semitransparent, light horn-colour, 

 or greyish, with sometimes a few darker zones and an irides- 

 cent hue, very faintly striated concentrically : epidermis 

 very thin : anterior and posterior sides cut off and sloping 

 from shoulders on the upper or dorsal side towards the 

 front margin, which is slightly curved and has sharp edges : 

 beaks central, very prominent, and capped with the fry or 

 nucleus of the shell, which is more globular than in the sub- 

 sequent 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 

 * Inhabiting lakes. 



