61. 
plate above mentioned, but quite detached from it. The 
second species so nearly resembles the fossil, that we 
can point out no difference ; they are both liable to the 
same variations of form, are of the same size, and both 
possess within the hinge the same kind of flat angular 
process ; a process that is attached in part to the line 
that supports the hinge cartilage, and in part to the be- 
fore-mentioned plate, from which it descends obliquely 
into the cavity of the valve. This appendage is regular 
and constant, and therefore should appear to be con- 
nected with the form of the animal ; whereas the second 
septum in M. polymorphus, does not always occur; and 
when it does, it is irregular and seems to be the effect of 
disease or over-luxuriant growth. 
This very interesting shell is abundant at Dax and 
Mérignac, and probably in several other parts of the 
Continent : but it was reserved for Charles Lyell, Esq. 
jun. to discover it in England. He found it in the ex- 
tensive bed of White Sand connected with the Lower 
Freshwater (or rather perhaps the so called Upper Ma- 
rine) Formation, in the Hordwell Cliff. It is accompa- 
nied by Mya plana (tab. 76.) in profusion, a Potamides 
like margaritaceus (tab. 339. f. 4.) a little Melanea? 
(Bulimus conicus? Brard), and, what is most curious, a 
small Serpula. We have therefore either a mixture of 
Marine and Freshwater shells in a bed hitherto thought 
by us to contain only Freshwater ones, or we are mis- 
taken in drawing conclusions from analogy without suf- 
ficient examination. ‘The Mya plana certainly resem- 
bles the Mya labiosa (a recent Freshwater species) more 
than it does any known Marine one: but it is slenderer, 
and the beaks are not eroded as in most Freshwater 
shells. 'The Potamides is an ambiguous genus. What we 
call a Melanea, and which Brard has referred to Bulimus, 
greatly resembles some species of Phasianella. The 
marks of distinction are small; as we are ignorant of 
the operculum, or if it ever had one. The Mytilus re- 
sembles more closely the African species, than that from 
the Danube; but the African species may be washed 
from the rivers down to the coast, or, like that from the 
Danube and the Wolga, it may be capable of living in 
