442 CYIMHNTD.'E. 



manifest upon the iimbonal region, and climinisliing more 

 rapidly and decidedly in front than behind. The surface, 

 which is of a reddish white, althonoh neither striated nor 

 grooved, is not smooth, being more or less wrinkled irre- 

 gularly in a concentric direction. Very fine and crowded 

 concentric wrinkles are also visible upon the thick epider- 

 mis which covers the entire exterior, and is of an uniform 

 more or less glossy fawn colour, becoming of a pitchy hue 

 in aged individuals. The ventral margin, which is inter- 

 nally simple and acute, is well arcuated throughout, and 

 ascends rather the more upon the anterior side. The front 

 dorsal edge, which at first subretuse, becomes eventually 

 convex or even arcuated, declines but moderately, not quite 

 rivalling the upward slope of the opposite lower margin. 

 The declination of the hinder dorsal edge is rather trifling 

 in the adult, but is proportionately more considerable in the 

 young ; this edge is much the more elevated, and except- 

 ing towards its termination is almost straight. The an- 

 terior extremity is narrow, and rounded both above and 

 below ; the posterior extremity is broad, and obtusely 

 rounded, or sometimes obsoletely sub-biangulated, the 

 lower angle being rounded off, and the upper being very 

 obscure, and often almost obsolete ; the middle of the pos- 

 terior edge is, however, not unfrequently (especially in the 

 young) straight and perpendicular, which is the chief cause 

 of the sh'ght angularity of that side of the sliell. The um- 

 bones are situated at about two-fifths the distance from the 

 commencement of the shell ; they are swollen, rather pro- 

 minent, and incline forward ; the beaks are acute, much 

 inflected, and not jireceded by any lunule, the area in front 

 of them is, however, much depressed. The ligament though 

 ample is not particularly projecting. The interior is of an 

 uniform chalky white, and is almost totally devoid of lustre. 



