SPHiENIA. 191 



Typically of a more or less rliomboitlal contour, this in- 

 teresting bivalve is wont notwithstanding to assume from 

 its terebrating powers, and its ability to adapt its shape to 

 the circumstances of habitation, very numerous modifica- 

 tions of its primary form. It is extremely inequilateral, 

 opaque, but not solid, and decidedly inequivalve ; the left 

 valve being both smaller and flatter than the right, a cha- 

 racter chiefly observable towards the lower margin and 

 posteriorly, the convexity (which is very moderate) being 

 almost equal at the umbones of either valve : these vary, 

 however, in elevation, that of the smaller being the less 

 projecting. Both valves are covered with a dull yellow 

 closely-attached epidermis, beneath which the surface is 

 nearly smooth or merely wrinkled with concentric, and 

 ordinarily rather distant, wrinkles of growth. The ventral 

 margin is usually more or less stralghtish, and generally 

 ascends a little behind ; the produced posterior dorsal edge, 

 which is a little retuse, runs nearly parallel to it ; the front 

 dorsal edge, which varies from almost straight to mode- 

 rately convex, declines so abruptly that the anterior side 

 appears almost truncated. 



The extremity of the remarkably short anterior side is 

 occasionally of an abbreviated wedge-shape, but more fre- 

 quently is rounded off below ; that of the elongated hinder 

 side is broad, and either biangulated or subrectangular 

 above, and more or less rounded off" at the lower angle, the 

 front edge being straightish or slightly convex, and but little 

 swerving from the perpendicular. The right umbo pro- 

 jects moderately above the dorsal line ; the beaks are 

 acute, and incurved ; in front of them lies a sort of lunule- 

 like depression, owing to the inflection of the valves at 

 that point. There is an umbonal ridge in both valves, but 

 it is more or less obtuse, and neither carinated nor sue- 



