ows no
special affinity to this genus.

5. PAECILASMA EBURNEA. Pl. II, Fig. 5.

TRILASMIS EBURNEA. _Hinds._ Voyage of Sulphur, 1844, vol. i,
Mollusca, Pl. xxi, fig. 5.

_P. valvis 3; scutis acuminatis, ovatis; ad pedunculum paene transverse
spectantibus; dentibus internis umbonalibus fortibus: tergis nullis:
carinae termino basali in discum amplum oblongum infossum producto._

Valves 3; scuta pointed, oval, placed almost transversely to the
peduncle; internal umbonal teeth strong: terga absent: carina with the
basal end produced into a large, oblong, imbedded disc.

Spines on the upper segments of the posterior cirri, arranged in three
or four approximate longitudinal rows, making small brushes.

_Habitat._--New Guinea, attached to the spines of a dead
Echinus. Brit. Mus., and Cuming.

_General Appearance._--Capitulum flat, pear-shaped, placed almost
transversely to the peduncle. Valves white, smooth, moderately thick.

_Scuta:_ the basal margin, as seen externally, is narrow, and can hardly
be separated from the carinal margin; but an internal basal rim, (fig.
5, _b_) (along which the imbedded disc of the carina runs,) shows where,
in the other species, the basal and carinal margins are separated. This
basal internal rim is not parallel to the external basal margin, but
runs upwards to the occludent margin, leaving beneath it a large
triangular space, to which the membrane of the peduncle is attached; and
this makes it appear as if the rostral umbones of these valves had grown
downwards; but, judging from the allied species, _P. fissa_, I have no
doubt that the primordial valves really lie on the umbones, and that the
growth has been in the usual direction, that is, exclusively upwards.
The occludent margin is curved, and blends by a regular sweep into the
carinal margin, so that there is no acute upper angle. A distinct line
can be seen, as if two calcareous valves had been united, running from
the umbo to the upper end of the valve, thus in appearance separating a
slip of the occludent margin; internally this appearance is more
conspicuous; this structure is important in relation to that of _P.
fissa_. The pointed umbones are divergent, and internally under each,
there is a large tooth. The two valves are equally convex.

_Terga_, entirely absent.

The _Carina_ (Tab. II, fig. 5, _a_, _c_), including the disc, is three
fourths as long as the scuta; it is placed almost transversely to the