424 MOLLUSCA. 
Arca sobria, Goutp; Proceed. Boston Soc. Nat. Hist., i. 278. 
July 1850. Expedition Shells, 85. 
SuHett of moderate size, transversely elongate, rhomboidal, rather 
thin, white; superior margin straight; anterior margin forming a 
right angle with the dorsal margin, then sweeping round in a regular 
curve into the ventral margin; posterior margin very obliquely trun- 
cate, joining the basal margin at an acute angle; ventral margin mo- 
derately gaping. Beaks large, elevated, nearly smooth; posterior 
slope angular. Surfaces sculptured with numerous and delicate ribs, 
rendering the surface somewhat nodulous ; behind the umbonal angle 
are five or six much larger bulging ribs, concentrically plicate, the 
intervening spaces smooth; at the anterior end are also some larger 
ribs. Cardinal area large, narrow-lanceolate, with grooves in lozenge; 
hinge edge delicate, with about eight delicate teeth at each end, and 
a few denticles between, the posterior one very oblique. Basal mar- 
gin finely crenulate. 
Length an inch and three-fourths ; height nine-tenths of an inch ; 
breadth seven-tenths of an inch. 
Habitat unknown. 
It has the outline of Arca Deshaysiz, but is much smaller, more 
slender, more numerously ribbed, less acute posteriorly. 
Figure 560, the side; 560a, the back; 560 3, the hinge. 
Nucuta sutcunata (Couthouy MS.) Gould. 
Testa transversa, inequilateralis, ovalis, nitida, concentricé sulculata, 
epidermide olivaceo induta ; natibus tumidis ; latere postico atienuato, 
sub-rostrato, viz truncato ; latere antico rotundato ; margine ventral 
arcuato ; declivitate postico angulato: lunula ovalis, bene impressa. 
Suet thick and solid, transversely oval, anterior extremity obtusely 
rounded, smooth ; posterior part attenuated, subrostrate, slightly trun- 
cated a little obliquely, with an obtuse ridge extending from the lower 
angle to the beaks; midway between this angle and the upper margin 

