SAXICAVA. 143 



shell, likewise, the downward inclination of the front dor- 

 sal margin is almost invariably arcuated or convex, whilst 

 in the present species it is oblique, and for a considerable 

 distance incurved, only becoming convex near its ventral 

 termination, which consequently is its most projecting part, 

 the chief prominence in rugosa being, on the contrary, usu- 

 ally situated nearer the dorsal side. 



The form is rhomboidal, with the length double or triple 

 the breadth of the shell, the former chiefly in the adult, the 

 latter frequently in the younger or immature individuals. 

 The dorsal and ventral margins are more or less subparal- 

 lel, and both exhibit a decided tendency to incurvation, 

 whenever permitted by the circumstances of growth to 

 assume an unrestricted outline. The valves are tolerably 

 strong (solid in aged specimens, which stage of growth 

 seems by no means common in collections), but rather fra- 

 gile in the young, opaquely white under a lighter or darker 

 brownish yellow epidermis, coarsely wrinkled concentri- 

 cally, and otherwise only marked with two widely diverg- 

 ing elevated lines which run from the beaks posteriorward, 

 one diagonally to the ventral side of the hinder extremity, 

 the other adjacent to (but not parallel with) the posterior 

 dorsal margin. These lines are armed (except in aged ex- 

 amples, where even the upper line itself is almost obsolete, 

 and the lower resembles an umbonal ridge,) with rather 

 strong and moderately sized scaly prickles, the narrow tri- 

 angular area between which series is usually slightly con- 

 cave. The front side is very short, occupying less more 

 frequently than beyond one quarter of the entire length ; 

 the hinder side is greatly produced, and abruptly (not 

 obliquely) subtruncated and bluntly biangulated at its 

 termination, which does not taper ofi:' as in the typical 

 rugosa^ but is the broader of the two extremities. The 



