al, of varying
proportions; the basal margin is generally narrow, and blends into the
carina-tergal margin; the internal basal rim generally is well
developed, sometimes with, and sometimes without internal teeth beneath
the umbones. In _P. eburnea_, and sometimes in _P. crassa_, there is a
line of apparent fissure, and in _P. fissa_ of actual disseverment,
running from the umbo to the apex of each scutum, nearly in the line in
which a ridge extends in Lepas: the primordial valves of the scuta in
these three species, are seated at the basal angles of the lateral and
larger segments. The positions of the primordial valves, and the
direction of growth in the calcified valves, are, in all the species,
the same as in Lepas. In several of the species attached to Crustacea,
the two scuta are unequally convex, which is caused, as was pointed out
to me by Mr. Gray, by that valve which lies close and nearly parallel to
the body of the crab, being least developed. The _Terga_ are either
quite absent, or rudimentary as in _P. crassa_, or pretty well developed
as in the other species: the occludent margin is single, and not double
as generally in Lepas; the basal angle is either pointed or truncated.
The _Carina_ varies considerably in shape, but never extends up between
the terga, nor ends downwards in a fork; in the first two species it is
truncated; in the others, it terminates in a deeply-imbedded oblong
disc, which in _P. eburnea_ seems almost entirely (but of course not
quite) to separate the inside of the capitulum from the peduncle; a
similar separation is effected in _P. fissa_, where the imbedded disc is
small, by two large teeth on the internal basal rims of the two scuta.
The carina is always narrow, and either solid internally or very
slightly concave.

_Peduncle_, is very short and narrow; the membrane is generally ringed
with thicker, yellower portions, and often bears very minute spines.

_Size._--All the species are small, with a capitulum not exceeding half
an inch in length.

_Filamentary Appendages._--None.

_Mouth._--Labrum generally considerably bullate in the upper part, with
a row of teeth on the crest. The _mandibles_ have four teeth, with the
inferior point narrow and spine-like, or rudimentary and absent. The
_maxillae_ have, under the two or three upper great spines, a deep notch
itself bearing spines; beneath this, the lower part is straight and
considerably prominent, Pl. X, fig. 15. Outer maxillae