462 MOLLUSCA. 
Diameters about two inches. 
Inhabits New Zealand. 
[t is impossible to declare that this shell has not been already de- 
scribed. It is difficult in this protean genus, to identify species even 
from good figures. As no figure yet published represents this species, 
and no description of any shell from the same locality answers to it, a 
representation and description are here given. O. denticulata, and O. 
spathulata, are most nearly allied to it. The excavated lower valve, with 
its deep umbonal pit, and its dark, slaty, frilled margin, and the oper- 
cular upper valve, with a few denticles near the hinge, are among its 
characteristic features. 
Figure 577, a group of shells ; 577 a, interior, and 577 8, exterior of a 
single specimen. 
OsTREA circumsuTA (Gould). 
T. solida, elongata, ovata, cinerea, inequivalvis, marginibus undulatis : 
valva superior denticulis radiantibus marginalibus in fovets sub-mar- 
gnahbus valve inferioris aptantibus: area cardinalis triangularis, 
contorta, longitrorsum fossata : intertor alba, limbo violacescente. 
Ostrea circumsuta, Goud; Proceed. Boston Soc. Nat. Hist., iii. 346. 
Dec. 1850. Expedition Shells, 96. 
SHELL small, elongated, narrow-ovate when growing freely, solid, 
ashy white, or sometimes with bronzed patches; structure dense; hinge 
area spirally curving to the right, broad, triangular, with a central 
channel in the lower valve; upper valve much smaller than the lower; 
at their point of contact is a series of radiating denticles around the 
whole margin, regularly spaced and of equal size, with corresponding 
pits in the lower valve, which on the left side, may be seen following 
round the margin of the hinge area, as also the hood-like tip of the 
upper valve; outside of this is a broad, somewhat undulated margin, of 
a slate-coloured or livid tint. Interior dead-white ; there are also groups 
of punctures on the side opposite the cicatrix. 
