30 



posterior end gibbous, the breadth and height about equal, and 

 equal to two thirds the length ; valves very convex, somewhat 

 angular longitudinally along the middle ; beaks acute, projecting, 

 remote, excurved ; ventral margin rectilinear or a little con- 

 cave ; hinge margin at first direct and then strongly arcuate ; dorsal 

 margin somewhat diverging from the ventral ; posterior end broadly 

 rounded ; hinge with two distinct denticles in one valve and three 

 in the other ; ligament excavation deep and broad, the inner plate 

 cribriform. Interior silvery on the disk, smoky black at the mar- 

 gins ; muscular and palleal cicatrices very deep ; limbus thick, 

 radiate striate. 



Largest specimens, length five eighths of an inch ; breadth and 

 height, three eighths of an inch. 



San Francisco. Maj. Rich. 



A very remarkable species clustering upon sea weed in the most 

 crowded manner, so that a string of them reminds one of a thickly 

 crowded cluster of bloom raisins ; so thick are they that the valves 

 of two adjacent species, not having room to open freely, wear each 

 other nearly through by the friction of opening and shutting. This 

 circumstance, together with the very strongly developed hinge and 

 deep cicatrices, forbid the idea of their being young specimens. 

 Byssus is very coarse, wax yellow. 



LITHODOMUS FALCATUS (PI. XVI. Fig. 9.) 

 Lithodomus fakatus, Gould; Proc. B. S. N. H., Nov. 1851. IV. 92. 



T. fragilis, margaritacea, subcylindracea, falcata, postice ad declivitatem 

 umbonalem valde angulata, epidermide castaned, rugis interdum bifurcatis cor- 

 rugata induta: apicibus ad octantem anteriorera positis valde involutis ; latere 

 antico dilatato, subgloboso ; latere postico arcuato, coarctato, acuminato. 



Shell fragile, falcate, subcylindrical, with a strongly marked 

 angle from the beaks to the base of the posterior extremity ; beaks 

 at the anterior eighth of the length, strongly involute and leaving a 

 deep depression in front of them. The anterior extremity is 

 rounded, dilated, broader than high ; the posterior end is arcuate 

 and acuminate ; against the beaks the valves are somewhat corn- 

 pressed ; the arcuation is produced chiefly by the deflection of the 

 superior margin, which is also rather sharp posteriorly. The sub- 

 stance of the shell is somewhat pearly when exposed by erosion, 



