394 MOLLUSCA. 
closely resemble the latter; the bilobed anterior lateral tooth of the 
right side, however, is quite peculiar. Small specimens remind one 
strongly of Mesodesma arctata and Jauresi. 
Figures 506, 506 a, lateral and dorsal views of the shell ; 506 4, inte- 
rior, with the hinge. 
Mactra DEBILIS (Gould). 
T. parva, alba, tenuis, transversa, elongato-ovata, antice rotundata, pos- 
tice acuta, valdé hians ; umbonibus antemedianis, eminentibus, acutis ; 
valvis tumidis, postice compressis, epidermide stramineo tenut indutis : 
cardo debilis ; dente V-formi parvo ; dentibus lateralibus curtis, acutis. 
Mactra debilis, Goutp; Proceed. Boston Soc. Nat. Hist., in. 348. 
December 1850. Expedition Shells, 98. 
SHELL small, white, very thin and fragile, inequilateral, widely 
gaping, transversely elongate-ovate, anteriorly regularly rounded, pos- 
terior extremity acutely rounded, somewhat nasute; posterior dorsal 
margin, very slightly arcuate; ventral curve regularly arcuate; beaks 
somewhat anterior, prominent, acute; valves somewhat flexuous; the 
disk tumid, compressed posteriorly, covered with a very thin, rugose, 
straw-coloured epidermis; very delicate concentric strie. Hinge very 
delicate ; V-tooth small; lateral teeth scarcely at all developed, very thin 
and short, between which, and the margin is a deep and long furrow, 
making almost the whole of the dorsal edge, as it were, two-edged. 
Length an inch and five-eighths ; altitude one inch ; breadth half an 
inch. 
Dredged at Singapore. 
A very delicate species, resembling in all its outlines small speci- 
mens of Mya arenaria. It affords a good example of the transition 
from Mactra to Lutraria, showing that there is no natural distinction. 
It is more widely gaping than most species. 
Figures 496, 496 a, lateral and dorsal views of the shell. 
