SCROBICULARIA. 439 



serted they are strongly wrinkled ; orifices plain : gills small, 

 triangular, symmetrical, hanging obliquely : palps of the same 

 size and shape as the gills, smooth outside and pectinated 

 within : foot proportionally large, muscular, slightly angulated 

 at the heel and granular at the point. 



Shell oval, rather convex, somewhat inequilateral, thin, 

 opaque, polished : sculpture, numerous slight, irregular, and mi- 

 nute concentric striae, crossed in some specimens (particularly 

 towards the beaks) by fine longitudinal lines, which appa- 

 rently pervade the whole fabric, as they are equally visible 

 on the inside surface ; the concentric striae occasionally di- 

 verge obliquely near the margin, as in S. prismatica ; lines of 

 growth distinctly marked : colour snow-white, with an opaline 

 hue : epidermis fibrous, pale yellowish- white : margins more 

 or less curved in front, rounded on the anterior side, bluntly 

 pointed on the posterior side, which is sometimes flexuous 

 and slightly angulated ; anterior dorsal margin gently sloping ; 

 posterior dorsal margin inclining from the beak at the same 

 angle, and expanding outwards where it meets the posterior 

 margin, so as to form a wedge-like edge ; the terminal angle 

 on that side is in a line with the transverse axis of the 

 shell: beaks blunt and incurved; umbones projecting: liga- 

 ment short, prominent, reddish-brown or horncolour : cartilage 

 curved, dark reddish-brown, constructed and placed like that 

 of S. prismatica : hinge-line obtusely angular : hinge-plate 

 thick and strong, occupying about one -fourth of the circumfer- 

 ence : teeth, in the right valve two minute erect cardinals on the 

 anterior side of the beak, besides a triangular lateral on each 

 side, the anterior one being double ; the left valve has only a 

 single small erect cardinal and a slight posterior lateral : in- 

 side nacreous and iridescent, finely and closely striated length- 

 wise and microscopically fretted ; margin indistinctly crenu- 

 lated : pallial scar having a short sinus of the same shape as 

 the shell, occasionally double in consequence of a shifting of 

 the base of the tubular fold : muscular scars usually shallow, 

 anterior triangular and elongated, posterior nearly circular. 

 L. 0-5. B. 0-8. 



Yar. 1. radiata. Shell of a thinner texture ; posterior side 

 marked with one or two clear rays. Syndosmya radiata, 

 Loven, Ind. Moll. Scand. p. 44. 



Var. 2. curta. Shell contracted at each end and conse- 

 quently more convex, rather solid; posterior side obliquely 

 truncate and distinctly angulated. 



