402 VENERIDiE. 



paler or darker rusty-brown, occasionally adornerl, but 

 chiefly in the younger examples, with rather indistinct 

 linear zigzag markings, or with about three obscure rays of 

 livid brown. When adult it is more or less ventricose, or 

 even inflated ; the convexity is broadly and tolerably evenly 

 diftused, although the valves arc slightly more compressed 

 anteriorly. The surface is covered with numerous concen- 

 tric much-elevated solid laminar ridges, which are closely 

 arranged, somewhat reflected or bending towards the beaks 

 (thus appearing hollowed out beneath, when examined 

 from above), and more or less distinctly indented at their 

 edges. These ridges at both ends, but far more evident- 

 ly behind, are broken up into rather large and horizontally 

 compressed wart-like tubercles, which form radiatingly 

 divergent rows. The interstices of the ridges are usually 

 traversed by one or two raised concentric lines, and to- 

 wards the umbones with depressed and rather distant 

 radiating ribs, which in some examples quickly become ob- 

 solete, but are continued in others almost to the base of the 

 shell. The ventral margin is more or less arcuated, and 

 rises far more in front than behind, thus attenuating below 

 the short and tolerably-rounded anterior side. The pos- 

 terior termination is rather broad, and very obscurely sub- 

 biangulated, the lower angle being almost entirely rounded 

 off in the adult, though sufficiently manifest in the imma- 

 ture individuals ; the posterior margin is more or less con- 

 vex. The front dorsal edge is short, and rather rapidly de- 

 clining, its slope is not incurved as the lunule pouts very 

 manifest at the lips ; the declination of the hinder dor- 

 sal margin is by no means considerable ; it is comparatively 

 rectilinear in the young, and becomes more curved in the 

 aged examples. The umbones are prominent, and, as well 

 as the beaks, incline much forward ; the latter are acute, 



