CARDIUM. 13 



more or less tinged with brown, and seated on a shallow 

 groove, which appears to connect them ; but in aged speci- 

 mens this is wont to become obsolete, and the knobs to be 

 converted into concentrically linear strong scale-like pro- 

 tuberances towards the ventral margin. This latter, in the 

 adult, is by no means arcuated in the middle, but curving 

 out and obliquely ascending in front, (far more than equal 

 to the downward inclination of the upper edge,) attenu- 

 ates below the rounded extremity of the anterior side. 

 It rises too posteriorly, but only sufficient to round off a 

 little the inferior angle of that side. The hinder margin, 

 although arcuated in the young, loses much of its con- 

 vexity, or becomes straightish and occasionally subretuse 

 in the mature shell, and is then moreover decidedly oblique. 

 The declination of the dorsal edges is extremely trifling ; 

 the front one, however, slopes the most ; their want of 

 convexity causes a more or less indistinct angulation at 

 the upper posterior corner, and prevents, likewise, the sym- 

 metrical rounding of the front one. The umbones are pro- 

 minent, and nearly central in the young, but in the adult 

 are distant from the anterior extremity only about two- 

 fifths of the entire length of the valves. The beaks are 

 greatly inflected, and slightly incline forward-; no defined 

 lunule precedes them, but there exists in the mature shell a 

 flattened area with subreflected lips, which is devoid of 

 costpe, and often stained with a darker tint. The ligament 

 is large and projecting : the umbonal slope is but little an- 

 gulated. Internally the colour is white, with occasionally a 

 rufous tint on the profundity of the valves, but no stain of 

 brown either upon the hinge-margin, or at the posterior 

 termination. The ribs on the inside extend only half way 

 to the beaks. The lateral teeth are moderately remote and 

 subequidistant ; the front one is large and strong. 



