BIVALVIA. 145 
both valves is furnished with two teeth, one simple, the other bifid, the simple one is 
placed before the umbo in the right valve, and the bifid one is anterior in the left, 
while the ligament occupies a position wholly external, and is deeply inserted : 
the muscle marks are large and well impressed, of an oblong form, with the mantle 
mark entire: numerous fine radiating strie are often visible in the interior, like some 
of the Lucine, the outside is what may be called smooth, having only the irregular 
lines of increase. The shell is somewhat flattened, though occasionally tumid, more 
especially on the posterior side. 
I have followed Philippi in assigning the Mediterranean shell to this species, as 
he has done in his second volume, the figure in the first volume more resembles the 
next species, for which it was taken when my Catalogue was compiled. 
2. DirpLoponta piLATaTA, S. Wood. Tab. XII, fig. a, 4. 
DirLoponta pitatTara. S. Wood. Catalogue, 1840. 
— — 1? WNyst. Coq. Foss. de Belg., p. 138, pl. 7, fig. 1, 1844. 
— — J. Sow., in Dixon. Geol. and Foss. of the Tert. and Cret. Form. 
of Sussex, p. 167, t. 3, fig. 16, 1850. 
Venus Fraaitis? Nyst and West. Nouv. Rech. Coq. Foss. d’Any., p. 9, pl. 3, fig. 11. 
Spec. Char. Testa transversd, ovatd, inflatd, inequilaterali, tenui, posticé longiore 
utringue convexd ; margine dorsali rotundato ; apicibus obtusis, depressis. 
Shell transversely ovate, tumid, inequilateral, thin, posterior side the larger, both 
sides convex ; dorsal margin rounded ; umbones obtuse, depressed. 
Length, ths. Height, 3ths of an inch. 
Locality. Coralline Crag, Sutton, and Gedgrave. Red Crag, Sutton. 
This species is not at all abundant. There are about a dozen specimens in my 
Cabinet presenting characters that appear of sufficient prominence to entitle it to be 
considered as different from the preceding one, and a few more particulars may therefore 
be pointed out to support the opimion. Our shell is more regularly rounded on both 
sides, and has not the squareness of outline so conspicuously shown in that species, 
where the dorsal margin forms a straight line, giving a distinct angle on the posterior 
side, whereas in this one it is eminently rounded, and the whole shell is more regularly 
tumid ; the posterior side is considerably the larger, and the umbones are rather 
depressed, turning a little towards the anterior, and the ligamental area is smaller than 
in the preceding species; the shell is thin, and the muscle marks not very well defined, 
but where they are seen, they appear to be different in size, the posterior one being the 
longer, and of a rounded oblong form, and that by the mantle without the least inflec- 
tion; the teeth are two in each valve, one simple, the other bifid, the posterior one 
is simple in the left valve, in the right it is anterior ; the bifid one is less, and the single 
one is better defined than in the preceding species ; the whole aspect of the shell is also 
different, that I have no hesitation in separating the two. ‘This species and rofundata 
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