Srom the Gulf of California, etc. 403 
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 (Pl. XVI. Fig. 9.) 
Lithodomus falcatus, Gould ; Proc. B. S. N. H., Nov. 1851. IV. 92. 
T. fragilis, margaritacea, subcylindracea, falcata, posticè ad declivitatem 
umbonalem valdè angulata, epidermide castaneâ, rugis interdum bifurcatis cor: 
rugatà induta: apicibus ad octantem anteriorem 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 com- 
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, 
