are covered on
their inner sides continuously with spines.

_Cirri._--The first pair is sometimes seated very distant from the
second. The arrangement of the spines on the posterior cirri varies, to
an unusual degree within the limits of the same genus. We have either
the ordinary structure of anterior pairs, with single fine intermediate
spines (as in _P. Kaempferi_ and _aurantia_), or we have the pairs
increased by one or two additional longitudinal lateral rows, as in _P.
eburnea_; or we have the front spines forming a single transverse row,
as in _P. crassa_ and _P. fissa_, Pl. X, fig. 29, _a_. The segments in
none of the species are protuberant; the anterior ramus of the second
cirrus does not seem to be thicker than the posterior ramus, as is
usually the case. The rami of the second, and of most of the other
cirri, are unequal in length,--the anterior ramus, contrary to the
ordinary rule, being longer in _P. eburnea_, _P. fissa_, and _P.
crassa_, than the posterior ramus by several segments; I have hitherto
observed this inequality only in the sessile genus Chthamalus.

The _Caudal Appendages_ are small, uniarticulate, and always furnished
with bristles.

_Distribution._--Four out of the five species live attached to
Crustacea in the European and Eastern warmer temperate and
tropical oceans; the fifth species was found attached to the
dead spines of an Echinus, off New Guinea. It is probable that
several more species will be hereafter discovered.

1. PAECILASMA KAEMPFERI. Pl. II, Fig. 1.

_P. valvis 5; carinae basi truncata et cristata: scutorum dentibus
internis umbonalibus fortibus: tergorum acumine basali truncato,
margini occludenti paene parallelo._

Valves 5; carina with a truncated and crested base; scuta with strong
internal umbonal teeth; terga with the basal point truncated, almost
parallel to the occludent margin.

Maxillae with short thick spines in the notch under the two upper great
spines; caudal appendages with scattered bristles on their summits, and
along their whole outer margins.

Japan; attached, in great numbers, to the upper and under sides
of the _Inachus Kaempferi_ of De Haan, a slow-moving brachyourous
crab, probably from deep water. British Museum.

_General Appearance._--Capitulum rather compressed, narrow, and
produced. Valves white, tinged with orange, smooth, moderately thin,
occasionally with faint traces of striae radiating from the umb