YENUS. 345 



at their extremities ; both have at their orifices fine red-brown 

 cirri, which in the branchial tube are alternately longer and 

 shorter : gills suboval, pale brown : palps triangular, striated 

 more distinctly on the inner than on the outer surface : foot 

 pure white, rather thick, slightly bent at the heel. 



SHELL triangular, rather convex, solid and opaque, more 

 glossy in young than in old examples : sculpture, regularly 

 imbricated concentric ribs, which vary much in number and 

 fineness, the course of imbrication being backward ; they are 

 now and then forked or become divided towards the posterior 

 end, and are crowded at each side and in front ; the whole 

 surface is also covered with extremely numerous and delicate 

 longitudinal striae which are only perceptible with the aid of 

 a strong magnifier : colour pale yellowish- white, usually va- 

 riegated by three reddish-brown longitudinal rays of dif- 

 ferent widths, composed of numerous and short broken streaks 

 or spots of the same tint, which sometimes cover the whole 

 of the shell and give it a beautifully stippled (or now and 

 then a vandyked) appearance; occasionally it is not varie- 

 gated, nor are there any coloured markings on the funda- 

 mental hue : epidermis extremely slight, only present on the 

 corselet and in young specimens : margins obliquely rounded 

 in front, curved upwards on the anterior side, and forming an 

 obtuse angle at that extremity in consequence of the excava- 

 tion of the lunule, produced and wedge-shaped on the poste- 

 rior side, which is somewhat flexuous and confined by an 

 acute angle formed by a junction with a long and nearly 

 straight slope on the dorsal side : beaks small, recurved and 

 separated ; umbonal area prominent : lunule varying in shape 

 from cordate to lanceolate, rather deep and defined by a dis- 

 tinct groove, longitudinally striated, sometimes tinged with 

 reddish-brown ; lips flexuous, much projecting : corselet broad 

 and somewhat deep, with shelving sides, obliquely striated : 

 ligament narrow, yellowish-brown, wholly exposed : hinge-line 

 obtusely angular : hinge-plate thick, broad, and flexuous : teeth, 

 in the right valve three divergent cardinals, the middle one 

 being somewhat conical and by far the thickest, the anterior 

 is the smallest but a little higher than the others, and the 

 posterior is very long, slight, laminar, and sometimes double ; 

 the left valve has also three cardinals nearly corresponding in 

 size and position with those in the opposite valve ; laterals 

 sometimes distinct, being apparently formed by the extension 

 of a ridge, which runs on the dorsal slope outside the posterior 



