{-'gfi PROCEEDINGS OF TEE XATIO3AL MUSEUM. VOL.XX. 



general outline of this end is broadly truncate; the ventral margin is 

 somewhat convex but slopes upward toward the posterior end and is 

 nearly straight along the middle; the posterior end is short, evenly 

 rounded, with the dorsal margin, behind the beaks, strongly incurved; 

 the large black ligament which occupies this area is continuous with the 

 epidermis, so that its outline forms a curve nearly in line with that of 

 the anterior end, and shows but a slight angle, or lobe, at the outer end 

 of the dorsal line. The uinbos are flattened, and scarcely prominent. 

 The whole surface is covered with a thick, smooth, glossy epidermis, 

 chestnut-brown in the young and brownish black in adult, which ante- 

 riorly is divided into several rather broad digitations that are shorter 

 and somewhat wider veutrally, their length diminishing from the mid- 

 dle of the anterior end to the ventral margin, along the middle of which 

 there are no digitations, but short and broad ones again appear pos- 

 teriorly. The shell is sculptured by radiating ribs and furrows which 

 are but slightly developed on the middle region but become large and 

 strong anteriorly and smaller and closer posteriorly. On the anterior 

 part these ribs are broad and flat, separated by wide, flat-bottomed, 

 furrows often nearly as wide as the ribs; on the middle area, the fur- 

 rows are shallow and rounded while the intervening spaces are flat, 

 sometimes broad, at others narrow, this region appearing comparatively 

 smooth, some specimens showing but slight indications of grooves and 

 ribs; posteriorly these are somewhat more numerous, narrower, often 

 about equal iu width. In specimens of medium size, there are from six 

 to eight of the large anterior furrows and as many ribs; the edges of 

 the latter are somewhat elevated above the middle portion and these 

 thickened margins extend out along the edges of the digitations which 

 otherwise conespond to the furrows. Internally the shell is white, 

 moderately thick, the anterior portion oblong with obtusely truncated 

 end, the dorsal and ventral margins nearly parallel, and the posterior 

 much narrower and tapered to an obtusely rounded end, with the 

 dorsal margin excavated for the ligamental area. The surface shows 

 distinct but not very prominent grooves and ridges corresponding to 

 the exterior ones; at the anterior end the margin shows slight lobes, 

 corresponding to the intervals between the epidermal digitations. The 

 anterior muscular scar is large and rounded, the posterior one is 

 smaller and subovate. Anteriorly the hinge-margin is thickened iu 

 both valves, running from near the beak nearly to the end; posteriorly 

 it is more strongly thickened by a sinuous callus to which the liga- 

 ment is attached, while under and just in front of the beak the margin 

 is excavated for the reception of the cartilage which continues forward 

 in a groove and is continuous with the broad, dorsal, epidermal mar- 

 gin which unites the two valves throughout their length. The posterior 

 ligament and anterior cartilage appear to blend just beneath the beaks; 

 the commencement of the cartilage is, however, indicated by a slight 

 notch in the call us- margin, iu both valves, and the ligament appears 



