glaringly incorrect to call a
five-valved species, a _Trilasmis_.

_Valvae, 3, 5, aut 7, approximatae: carina solum ad basales apices
tergorum extensa, termino basali aut truncato aut in discum profunde
infossum producto: scuta paene ovalia, umbonibus ad angulum rostralem
positis._

Valves, 3, 5, or 7, approximate: carina extending only to the basal
points of the terga; with its lower end either truncated or produced
into a deeply imbedded disc. Scuta nearly oval, with their umbones at
the rostral angle.

Mandibles with four teeth; maxillae notched, with the lower part of edge
prominent; anterior ramus of the second cirrus not thicker than the
posterior ramus; caudal appendages uniarticulate, spinose.

Generally attached to Crustacea.

I have already given my reasons for instituting and separating this
genus from Lepas; as far as the capitulum is concerned, the differences
between these genera certainly appear trivial; they consist in the
carina not extending up between the terga, and in the lower end being
either truncated, or produced into an imbedded disc: the terga have a
single occludent margin. The included animal's body differs in more
important respects; for both mandibles and maxillae are very distinct;
the cirri of some of the species also differ; and the caudal appendages
are here always spinose: there are no filamentary appendages: and
lastly, the habits are different.

The genus may be divided into two sections, firstly, _P. Kaempferi_ and
_P. aurantia_, which have their carinae basally truncated, the basal
angles of their terga cut off, and the anterior rami of their second
cirri shorter than the posterior rami; and, secondly, _P. crassa_, _P.
fissa_, and _P. eburnea_, which in these several respects are otherwise
characterised. The _P. eburnea_, however, differs rather more from _P.
crassa_ and _P. fissa_, than these two do from each other; but certainly
not enough to allow of the retention of Mr. Hinds' genus of Trilasmis.
_P. crassa_, in an especial degree, connects together all the forms.

_General Appearance._--Capitulum oval, more or less produced, flat or
gibbous; formed of three, five, or seven approximate valves; the lesser
number arising from the abortion of the terga, and the greater number
from the scuta being divided into two segments. Valves moderately thick,
either white or reddish, smooth or striated, and sometimes partly
covered by membrane, bearing minute spines. _Scuta_ ov