50 LUCINID^. 



most decidedly exceeds the breadth in all the numerous 

 examples we have hitherto observed. The valves are 

 rather strong, opaque, and of an uniform lustreless white; 

 when young- they are compressed, but in the more aged 

 individuals are subcentrally (or rather higher than the 

 middle of the disk) almost ventricose, rapidly diminishing, 

 however, in convexity towards the margins. The surface 

 is concentrically wrought with numerous extremely deli- 

 cate laminar striae, which typically are almost equidistant, 

 and cover the entire area, but in some examples approxi- 

 mate towards the lower margin, or become obsolete upon 

 the more swollen portion of the disk ; occasionally too 

 they are extremely fine and depressed, and most crowdedly, 

 and sometimes irregularly disposed. These membranaceous 

 lines (whose interstices are not visibly decussated), ab- 

 ruptly ceasing near the hinder dorsal edge, surround the 

 smooth and flattened or slightly excavated lozenge (which 

 is lanceolate, and pointed at both extremities) with a 

 crest of small spinous protuberant scales formed in the 

 younger specimens, and in the upper portion of the adult 

 ones, by the confluence of each pair of striae at their ter- 

 mination. The ventral margin, which is internally some- 

 what flattened and devoid of crenation, is moderately 

 arcuated, and rises the more in front ; its chief swell is a 

 little before the middle. The dorsal edges vaiy from 

 rather slightly to moderately sloping ; the hinder one is 

 convex, the front one is decidedly incurved. The um- 

 bones project but little, and are usually rather compressed 

 towards the very small beaks, in front of which lies a rather 

 large lanceolate sunken smooth-surfaced lunule. The ante- 

 rior side is rather the shorter ; its extremity, "VA'hich is sub- 

 angulated above, and well rounded below, is about equal 

 in breadth to the subtruncated posterior termination ; the 



