ven of the much simpler case of
the mere separation of the sexes in _Ibla Cumingii_: nor can any
explanation, I believe, be given of the much more varied arrangement of
the parts of fructification in plants of the Linnean class, Polygamia.

_Genus_--SCALPELLUM. Pls. V, VI.

SCALPELLUM. _Leach._ Journ. de Physique, t. lxxxv, July, 1817.

LEPAS. _Linn._ Systema Naturae, 1767.

POLLICIPES. _Lamarck._ Animaux sans Vertebres, 1818.

POLYLEPAS. _De Blainville._ Dict. des Sc. Nat., 1824.

SMILIUM (pars generis). _Leach._ Zoolog. Journal, vol. 2, July,
1825.

CALANTICA (pars generis). _J. E. Gray._ Annals of Philosophy, vol.
x, (new series,) Aug. 1825.

THALIELLA (pars generis). _J. E. Gray._ Proc. Zoolog. Soc., 1848.

ANATIFA. _Quoy_ et _Gaimard_. Voyage de l'Astrolabe, 1826-34.

XIPHIDIUM (pars generis). _Dixon._ Geology of Suffolk, 1850.

(_Herm. et Foem._) _Valvis 12 ad 15: lateribus verticilli inferioris
quatuor vel sex, lineis incrementi plerumque convergentibus: sub-rostrum
rarissime adest: pedunculo squamifero, rarissime nudo._

(Herm. and Fem.) Valves 12 to 15 in number: latera of the lower whorl,
four or six, with their lines of growth generally directed towards each
other: sub-rostrum very rarely present: peduncle squamiferous, most
rarely naked.

Filamentary appendages, none: labrum, with the upper part highly
bullate: trophi, various: olfactory orifices, more or less prominent:
caudal appendages, uniarticulate and spinose, or none.

_Males_, parasitic at or near the orifice of the sack of the female or
of the hermaphrodite: thorax enclosed within a capitulum, furnished with
three or four rudimentary valves, or with six perfect valves: peduncle
either short and distinct, or confounded with the capitulum: sometimes
mouth and stomach absent, and cirri non-prehensile; sometimes mouth and
cirri normal.

Generally attached to horny corallines, in the warmer temperate
seas over the whole world.

I have felt much doubt in limiting this genus: the six recent species
which it contains, differ more from each other than do the species in
the previous genera. Mr. Gray has proposed or adopted generic names for
four of the species, and a fifth certainly has equal claims to this same
rank. These genera have been founded almost exclusively on the number of
the valves; and oddly enough, the numbers have generally been given
wrongly, namely, in Scalpellum, Calantica, Thal