13 
CORBULA. Bruguiére. La Marck. 
Gen. Cuar. Shell with two dissimilar unequal- 
sided valves; sub-transverse. Hinge teeth 
two, one in each valve ; ligament internal. 
Oxe of the most easily defined genera, being well dis- 
tinguished by the dissimilarity of the valves, of which one 
is generally so small as to he included in the other, and 
by the single recurved conical tooth in the larger valve, 
and a corresponding pit with an irregular tooth in the 
other: both valves are more or less gibbose, with in- 
curved beaks, a projecting and sometimes recurved an- 
terior side. Several species, both fossil and recent, are 
described ; some are slender, others very stout shells. 
CORBULA gigantea. 
TAB. CCIX.---Figs. 5, 6, and 7. 
Spec. Cuar. Gibbose, when young suborbicular, 
when old transversely oblong ; anterior side 
produced, recurved; surface concentrically 
furrowed near the beaks ; posterior side beset 
with short spines. 
Y ovune shells have the furrows and ribs between, which 
are equal and very regular, extended over the whole 
surface of both valves ; they are broadest over the middle, 
and gradually diminish to the sides: as the shell increases 
in size, the ribs are closer, and less prominent towards 
the front, and at length disappear: when full grown, 
short obtuse spines placed in longitudinal rows otcupy 
