BIVALVIA. 63 
tripartite division of the exterior is of unequal dimensions, the posterior strize covering 
nearly but not quite half the surface, while the anterior occupies rather a less space than 
the centre or naked compartment; the striz or rays are large and rounded, number- 
ing about a dozen or fourteen on the anterior side, with about double that number on 
the posterior portion: the whole shell is covered with transverse striz or regular 
lines of increase, which prettily ornament the spaces between the ridges, and 
the edge of the shell is deeply crenulated on the anterior and posterior sides, or 
those portions which are covered with the radiating ridges; the tripartite division 
of the shell, is visible in the interior, and the number of the external striz may be 
counted there. The edge of the ventral margin has a slight convexity, contracting a 
little towards the striated parts. 
The differences between this species and the following are so evident, there 
can be no mistake, that shell being more compressed or less tumid, with the posterior 
side broader in proportion. The shell to which this approaches nearest, is 
M. semi-nuda, Desh., ‘Desc. des Coq. Foss. des Env. de Par.,’ vol. i, p. 264, pl. 30, 
figs. 20—22, a fossil belonging to the Formations of the Older Tertiaries. I have not 
been able to obtain a specimen from the Paris Basin; but what I presume to be the 
same species in the Cabinet of Mr. Edwards, from the English Deposits, presents 
differences that may be regarded as specific. Mr. Edwards’s shell is more regularly 
ovate, and is even thinner than our species, with fewer radiations on the anterior 
side, not having more than seven or eight, and these are broader, it is also, more 
regularly tumid than our own shell, which has somewhat of an obtuse angle on the 
posterior portion. 
The shell figured and described by Dr. Gould, under the name of JZ. discors, seems 
to present but trifling differences with the British shell, judging from description alone ; 
but it is considered to be distinct by British Conchologists, as well as by Dr. Lovén. 
7. Mopioxa piscors, Limneus. Tab. VIII, fig. 5. 
Myritus piscors. Linn. Syst. Nat., ed. 12, p. 1159, No. 261, 1767. : ‘ 
—  DIscREPANS. Mont. Test. Brit., p. 169, 1803. 
Moprota piscrEepans. Turt. Brit. Biv., p. 202, 1822. 
— — Desh. 2d ed. Lam., t. vii, p. 23, 1835. 
— — Forbes. Malac. Monens., p. 44, 1838. 
— — Miller. Ind. Moll. Groenl., p. 19, 1842. 
— — Alder. Cat. Moll. North. and Durh., p. 81, 1848. 
Moproiarta piscors. JLovén. Ind Moll. Scand., p. 33, 1846. 
CRENELLA  iscors. For, and Hanl. Hist. Brit. Moll., vol. i, p. 195, pl. 43, figs. 4, 6, 
and pl. 48, fig. 5, 1849. 
Spec. Char. Testa ovato-ellipticd, subcompressd, valdé inequilaterd, tenui ; anticé et 
postice striata, spatio mediano levigato ; latere postico latiore. 
Shell ovato-elliptical, somewhat compressed, very inequilateral, thin; striated at 
