us_.

In the characters of the mandibles, and more especially of the outer
maxillae; in the length of the first pair of cirri; in both rami of the
second and third cirri having their basal segments brush-like, with
pectinated spines; and in the shape of the caudal appendages, there is a
close relationship to _P. spinosus_, and through this species to
_Scalpellum villosum_. In the little prominence of the olfactory
orifices, P. _sertus_ differs from most of the allied forms, excepting
_P. spinosus_. In the maxillae having two prominences bearing fine tufts
of bristles, in the roughened knobs on the prosoma, and in the presence,
in some individuals, of two or three whorls of valves under the carina
and rostrum, there is a marked tendency in _P. sertus_ to approach _P.
cornucopia_, _P. elegans_, and _P. polymerus_.

_Genus_--LITHOTRYA. Pl. VIII, IX.

LITHOTRYA. _G. B. Sowerby._ Genera of Shells, April 1822.

LITHOLEPAS. _De Blainville._ Dict. des Scienc. Nat., 1824.

ABSIA.[65] _Leach._ Zoological Journal, vol. ii, July 1825.

BRISNAEUS et CONCHOTRYA. _J. E. Gray._ Annals of Philosophy, vol.
x, (new series,) August 1825.

LEPAS. _Gmelin._ Systema Naturae, 1789.

ANATIPA. _Quoy_ et _Gaimard_. Voyage de l'Astrolabe, 1832.

[65] The description of Absia is so inaccurate, that I should not
have recognised it, had not the _Lithotrya Nicobarica_, in a
bottle in the British Museum, borne this name.

_Valvae 8, si inter eas parvum (saepe rudimentale) rostrum et duo parva
latera numerentur; incrementi lineis concinne crenatis: pedunculus
squamis calcareis parvis vestitus, in verticillis superioribus crenatis;
aut calyci basali calcareo aut discorum ordini affixus._

Valves 8, including a small, often rudimentary rostrum and a pair of
small latera: lines of growth finely crenated. Peduncle covered with
small calcareous scales, those of the upper whorls crenated; attached
either to a basal calcareous cup, or to a row of discs.

Body lodged within the peduncle: mandibles with three teeth, the
interspaces being pectinated; maxillae various: olfactory orifices
slightly prominent: caudal appendages multiarticulate.

Lodged in cavities, bored in calcareous rocks, or shells, or
corals; generally within the Tropics.

_Description._--The capitulum is not much compressed, a horizontal
section giving an oval figure; it is placed obliquely on the peduncle,
the scuta descending lower than th