ACEPHALA. 397 
superior dorsal edge strongly concave; a considerable ridge passes 
from the beak to the lower posterior angle; ventral edge strongly 
arched; hinge without a tooth, but with a deep, oblique ligament- 
groove, and an anterior marginal rib. 
Length an inch and an eighth; breadth half an inch; height an 
inch and one-tenth. 
Inhabits Patagonia. 
More solid, more concave, and more elongated than any species I 
am acquainted with. The upper valve of a small specimen from the 
same locality is semicircular, but more acute anteriorly than poste- 
riorly. 
Figure 500, the shell open, with the hinge; 500 a, dorsal edge of 
the shell; 500 8, flat valve. 
OsTEODESMA BRACTEATUM (Gould). 
Testa parva, tenuis, transversa, inequilateralis, ovato-falcata, ventricosa, 
margaritacea, epidermide fusco radiatim creberrimé rugoso induta ; 
latere antico rotundato ; latere stphonali duplo longiore, triangulart, 
sub-adscendente, truncato; margine dorsal recto ; margine ventrah 
arcuato, sub-pendulo : interior deaurata. 
Osteodesma bracteata, GouLp; Proceed. Boston Soc. Nat. Hist., iii. 
217. May 1850. Expedition Shells, 77. 
SHELL extremely thin and fragile, translucent, pearly, nacreous and 
reflecting golden colours internally, and somewhat externally ; form 
oblong-ovate, inequilateral, somewhat recurved; beaks acute, at the 
anterior third; anterior end obtusely rounded; posterior end trian- 
gular-elliptical, broadly truncated at tip, the basal portion, where the 
umbonal slope attains, being notably ventricose: basal margin at this 
part depending ; anterior dorsal edge arcuate; posterior dorsal recti- 
linear, compressed. Surface covered with a dusky epidermis of con- 
siderable firmness, which is corrugated into numerous, delicate, radia- 
100 
