LUTRARIA. 371 



although by no means solid for their size, yet sufficiently 

 strong, almost oj^aquo, and decidedly shallow. This com- 

 pression is very manifest upon the umbonal region, from 

 whence to the ventral margin there is often indeed a slight 

 retusion of the external surface : the hiation is very con- 

 siderable, the valves merely touching each other at the 

 beaks and along the middle portion of their lower margins, 

 and gaping more particularly at the hinder extremity. 

 The outer surface is nearly smooth, or at most concen- 

 trically wrinkled by coarse irregular lines of growth ; it 

 is of a squalid white, or stained with a reddish rust colour, 

 covered, when recent, with a thin but rather tenacious epi- 

 dermis, of an ashy olive colour (almost passing into drab 

 in a foreign variety), which is smoothly laid on, and varies 

 in lustre from dull to moderately glossy. The ventral 

 margin is almost straight or somewhat retuse in the 

 middle ; from thence becoming convex on either side, it 

 ascends very obliquely and considerably in front, rising far 

 more moderately behind. The anterior side is much the 

 smaller, occupying, in some examples, not more than one- 

 half the entire length of the shell : it is liable to great 

 variation in shape from the absence or existence of depres- 

 sion at its upper or dorsal margin. This edge, in very large 

 individuals, where the lips are apt to expand, or become in 

 a degree subreflected, is convex and moderately sloping; 

 but in the majority of specimens (those of average size), 

 is more or less retuse, and not much declining ; in ' the 

 former case, the lower corner of the front extremity being 

 invariably rounded off, there is only a slight subcentral 

 angulation at the anterior end ; but in the latter event, a 

 distinct upper angle. The posterior termination is rounded 

 both above and below, yet not equally so, the chief swell 

 of the hinder outline being rather above the middle, as the 



