464 MOLLUSCA. 
I have seen but one specimen of this delicate and well-formed 
oyster; and notwithstanding the usual inconstancy of form in the 
species of this genus, I think its characters are so diverse from any 
hitherto published, that it may be safely pronounced undescribed. 
The small, purple-striped, West India oyster (O. Brastlana), is simi- 
lar to it. 
Figures 578, 578 a, views of the exterior and interior of the shell. 
OsTREA MoRDAX (Gould). 
T. petrosa, angusta, lunata: valva inferior concava, digitis triangulart- 
bus erectis fuscescentibus marginata : valva superior minor, planulata, 
margine profunde sinuato et granulato, sinibus cum digitis coaptanti- 
bus: interior albida, olivaceo-maculata ; cicatrice elongata, valde in- 
crassata, nigra: area cardinalis planulata, lata. 
Ostrea mordax, Goutp; Proceed. Boston Soc. Nat. Hist., 11. 346. 
Dec. 1850. Expedition Shells, 96. 
SueLt small, very solid, narrow and elongated, spatulate, curving 
a little to the right, of a very condensed structure, showing vestiges 
of a thin, olivaceous epidermis between each layer, when abraded, 
and marked with very close-set lines of growth. Lower valve con- 
cave; upper valve flat, much smaller, and setting deeply into the 
lower. Its edges are scolloped with numerous rounded sinuosities 
and digitations, which are adapted to corresponding erect, triangular, 
tooth-like folds in the lower valve: these digitations are mostly stained 
with sienna-brown: on the inner side, opposite to the sinuations, and 
close upon them, are coarse, compressed, granular denticles, perpen- 
dicular to the margin, quite large near the hinge, and extending out 
each side of the hinge area, but growing smaller and more numerous 
towards the tip; a corresponding series of pits in the under valve. 
Interior dingy white, with olive or sienna stains; cicatrix elongated, 
black, very thick, and becoming quite elevated at its ventral end; hinge 
area flat, broad, a little tortuous. 
Length two inches; breadth one inch. 
