. Bennett, I believe, in the Atlantic, in which the
extreme narrowness of the carina and of the terga (Pl. I, fig. 6, _b_,
_c_) (with consequent wide spaces of membrane left between these
valves), led me, at first, to entertain no doubt, that it was quite a
distinct species, which was strengthened by finding that the whole
surface of the cirri were villose, with very minute spines; hence I
called this variety, _villosa_. On the closest examination, however, I
could detect no other differences, and the narrowness of the carina and
terga varied very considerably: moreover, in one of the specimens, which
was about intermediate in the form of its valves between this variety
and the common form, the surfaces of the cirri were not in the least
degree villose. Again, in some other specimens, the terga were as narrow
as in Mr. Bennett's, whilst the carina had its usual outline.

In a var. (called by Leach, _P. Donovani_,) from the Atlantic, under the
Equator, the carina is remarkable from the extreme flatness of the upper
part, and from the presence of an exterior, narrow, central ridge. In
one specimen from Jersey, in the British Museum, the carina made an
extremely near approach to this same form.

_Affinities._--This species is certainly much the most distinct of any
in the genus, and Mr. Gray has proposed to separate it under the name of
Dosima; but considering the close similarity of the whole organisation
of the internal parts, together with the transitional characters
afforded by _L. australis_, I think the grounds for this separation are
not quite sufficient. I have remarked, under _L. australis_, on the
affinity between that and the present species. In the carina terminating
in a disc (though here not imbedded), there is some slight affinity to
_Paecilasma eburnea_ and _crassa_, and markedly so in the arrangement of
the bristles on the posterior cirri. In the valves being covered with
villose membrane, and to a certain extent in the form of the carina and
of the occludent margin of the terga, and especially in the two rows of
cement-orifices in the peduncle, there is some affinity to Scalpellum.

PAECILASMA. _Nov. Genus._[30] Plate II.

ANATIFA. _J. E. Gray._ Proc. Zoolog. Soc., 1848, p. 44.

TRILASMIS. _Hinds._ Voyage of the Sulphur. Mollusca, 1844.

[30] [Greek: Pokilos], various, and [Greek: elasma], plate or
valve. I have not been able to adopt Mr. Hinds' name for this
genus, as it would be too