s much
harder and more persistent; and as the whole valve is formed of
component laminae thus edged (the once continuous laminae of non-calcified
chitine connecting the valves, having disintegrated and disappeared) the
surfaces of the valves are generally left covered by a persistent
membrane, constituted of these edgings: this membrane has been called
the epidermis. In some genera, as in Lepas, this so-called epidermis is
seldom preserved, excepting on the last zone of growth: in Scalpellum
and Pollicipes it usually covers the whole valves. It appears to me that
the laminae of chitine, and of calcified chitine composing the valves,
are both formed not by secretion, but by the metamorphosis of an outer
layer of corium into these substances.

[13] Chitine is confined to the Articulata. It was Dr. C. Schmidt
(Contributions, &c., being a Physiologico-Chemical investigation:
in Taylor's 'Scientific Memoirs,' vol. v), who discovered that
the membrane connecting the valves and forming the peduncle, and
the tissues of the internal animal, were composed of this
substance. But Dr. Schmidt says that the valves in Lepas are
composed of 3.09 of albuminates, and 96.81 of incombustible
residue; I cannot but think that the existence of the albuminates
is an error caused by Dr. Schmidt's belief that the Cirripedia
were intermediate between Crustacea and Mollusca, in the shells
of which latter, the animal basis consists of albuminates. For
after placing the valves of Lepas and Pollicipes in cold acid, I
found that the membrane left could _not be dissolved_ in boiling
caustic potash, but could, though slowly, (and without change of
colour,) in boiling muriatic acid; and these are the main
diagnostic characters of Chitine, compared with albuminous
substances. I may add, that Schmidt was also induced to consider
the shells of Cirripedia as having the same nature with those of
Mollusca, from finding that in the above 96.81 of incombustible
matter, 99.3 consisted of carbonate and only 0.7 of phosphate of
lime; but Dr. Schmidt's own analyses prove how extremely variable
the proportions of these salts are in the Crustacea, as the
following instance shows:--

_Lobster._ _Squilla._

Phosphate of Lime 12.06 47.52
Carbonate of Lime 87.94 52.48

And, therefore, it is not very surprising that Cirripedia should
have st