384 • A'ENERID^. 



The general outline is subrliomboiclnl, and suboval (typi- 

 cally it is more elongated than the preceding) ; the valves 

 are moderately strong, opaque, very inequilateral, and more 

 or less ventricose, the convexity being more evenly diftnsed 

 than in decussata. The outer surface is almost entirely de- 

 void of lustre, and is usually of a chalky or dirtj'-white, 

 which shades into a browner tint in the adult ; this is very 

 rarely destitute of markings, being almost always diversified 

 by radiating angulated spots, and zigzag confluent splotches 

 of a livid brown ; occasionally the entire surface is finely reti- 

 culated, (though usually but faintly so,) with angular lines 

 of that hue, but the colouring matter is generally more 

 lavishly displayed on the posterior extremity, to which 

 portion occasionally the pencilling is confined. Crowded 

 and wavy stria?, which are coarser at the extremities, and 

 of which the interstices have a tendency to become imbri- 

 cated and lamellar at the posterior end, traverse the entire 

 shell in a concentric direction, and are most finely decus- 

 sated by radiating striukc, which are very closely disposed, 

 and become obsolete towards the beaks. The ventral mar- 

 gin is convex, but usually inclines to straightness near the 

 middle, though rising greatly and arcuatedly in front, and 

 moderately so behind. The anterior side is very short, 

 occupying but little more than one-fourth of the entire 

 length ; its extremity is very narrow and obliquely rounded 

 below, but is very slightly angulated above, owing to the 

 straightness of its extremely short dorsal margin. 



The termination of the produced posterior side is sub- 

 biangulated and broad, the convex, or subarcuated hinder 

 margin, which is but slightly oblique, forming an obtuse 

 angle with the almost rectilinear and scarcely in the least 

 declining dorsal edge ; the lower angle is rounded off. The 

 ligament is tolerably large, but not at all elevated ; the 



