Pvn H,i I'UitlS

412 MACTRID.E.
however, a numerous and widely spread group
: its
favourite habitat is sand or mud. Dr. Carpenter has
ascertained that the texture of the shells presents the
same general characters as in the Tellinida?, but that the
indications of organic structure are more distinct. In
those of Mactra and Lutraria the cells are especially
observable, although irregular in the one and elongated
in the other.
Genus I. AMPHIDES'MA*, Lamarck. PL VIII. f. 1.
Body oval, compressed : mantle thick, united for two-thirds
of its length : tubes separate and unequal, not very long, and
having their orifices fringed
with cirri.
Shell oval, only slightly inequilateral, wedge-shaped, closed
at both ends, longitudinally as well as concentrically striated
in some : species beaks turned towards the posterior side : teeth,
two cardinals, of unequal size, in each valve ;
less distinct.
laterals more or
Although, as Lamarck observed, the Amphidesmata
in the aggregate appear to form an artificial — group
and, as originally constituted, they were truly
a hete-
rogeneous assemblage of species — the characters of this
and it serves to
genus in a restricted sense are peculiar,
connect the present with the immediately preceding
family. In the outward form of the shell (as well as
in the siphonal tubes) it resembles Donax, and in the
structure of its hinge Mactra. Whether the genus
Mesodesma of Deshayes is different may be an open
but there is much less cause to separate Tur-
question ;
ton's genus Ervilia, which not only was very inade-
quately and inaccurately defined, but did not even include
the only British species lately assigned to it by
Forbes and Hanley, and which will be now described.
* Having a double ligament.