33 
above the elongated socket d (PL X, P'ig. 2). The shell is therefore 
oj)istliodetic. There is also a well marked nymph f (PI. X, P'ig. 2), widening 
in a jiosterior direction. 
The luniile, although not large, comparatively speaking, is a broad, 
somewhat shallow excavation well in advance of the umbo. Both beneath 
and slightly anterior to the iimho is an area vt (PI. X, Fig. 2), cordate in outline, 
moderately concave, traversed by two obliquely transverse grooves / and k 
(PI. X, Fig. 2), both superficial, and hounded posteriorly by a third g (PI. X, 
Fig. 2), deep and of mneh morphological interest. This third groove i)rocceds 
from below the nmhonal apex, almost at right angles to the hinge plate direct 
to the cardinal tooth a (PL X, Fig. 2), almost bisecting it, passing between 
and separating the sockets h and c (PL X, Fig. 2), hut beneath the nmhonal 
apex it unites with the ligament groove proper, h (LI. X, Fig. 2). 
The adductor impressions (PL IX, Fig. 1) are well developed, although 
not so large as one would have expected in a shell of this size and thickness ; 
at any rate, the anterior is not. The latter is situated immediately at the 
anterior end of the lunule, high upon the compressed anteidor end, near the 
cardinal margin. The posterior scar is proportionately much larger than the 
anterior. 
The pallial line leaves between it and the ventral margin a comparatively 
narrow inter-marginal area (PL IX, Fig. 1), and describes an upwardly 
directed obtuse curve, immediately heloAV the posterior adductor scar, and in 
joining the latter leaves the shallowest of sinuses ; there is no pallial tongue. 
I have, so far, detected only feeble pedal scars, for possibly the foot of this 
mollusc was not highly developed. 
Fissilunula,hvou,(\\y speaking, seems to he an ally of the Isoconlia group, 
hut generally does not agree Avith any memher of that family known to me. 
For the purpose of comparison I have inserted outline drawings of the articuli 
of Isocardia humana, Linn. {= I. cor, Linn, et auct.), and I. lunnlata, Xyst., 
and although there is a general faudamental resemblance between the hinge 
plates of all three forms, yet, when examined in detail, part by part, many 
points of difference are observable. The lunules of the Ptecent and Tertiary 
species are traversed obliquely by two impressed grooves o“'^, (PL X, Fig. 
3; PL XI, Fig. 3), and hounded posteriorly by a long curved groove, 
(PL X, Fig. 3 ; PL XI, Fig. 3), immediately under the inrolled umhones. The 
lunule in I. lunnlata is much deeper and more marked than in I. humana, and 
E 
