CHITON.—Ptats I, 
The animal Chiton is an oblong hirudiniform mass with- 
out eyes, and possessing a thin membranous veil in place 
of tentacula over the mouth, which has a long spiral 
tongue fwmished with horny denticles. The respiratory 
organs, selected to characterize the subdivision of the Gas- 
tropods into Orders, consists of a series of pyramidal leaf- 
lets which encircle the body, as in Patella, within a de- 
pression between the edge of the mantle and the foot. 
The anal aperture is situated quite at the posterior ex- 
tremity; and the Chiton, possessing a double system of 
generation is hermaphroditic. The arrangement of the 
branchize is therefore similar to that in Patella, and the 
two genera were accordingly associated by M. Cuvier in 
a particular order with the title Cyclobranchiata, as the 
lowest form of the Gastropod type. The chief peculiarity 
of this mollusk, and in which it differs in a remarkable 
degree from Patella, consists in the circumstance of its 
secreting the shell in eight separate pieces, sustained in 
order by a horny expansion of the mantle, and moving 
upon each other, after the manner of plate armour, by 
the aid of three flexible muscles attaching crosswise to 
each plate or valve. This horny expansion of the mantle 
is sometimes only marginal, constituting a frame to the 
shell, sometimes partially or entirely spread over the shell ; 
and it is characterized by the following variety of orna- 
ment. In C. elegans the horny ligament appears in its 
simplest form, thin and transparent; in C. alatus it is 
covered with a rough arenaceous surface ; in C. Blainvillir 
with a few scattered very short bristles; in C. spiniger 
with thickly-set calcareous bristles ; in C. Perweianus with 
a dense growth of hair; in C. setiger with irregular strag- 
gling hairs; in C. Coguimbensis with curious oblong pro- 
cesses, peculiar to that species alone; in C. aculeatus with 
rude short cylindrical spines; in C. spinosus with sharp 
curved spines; in C. magnificus with close-set calcareous 
grains; in C. sguamosus with equally close-set scales; in 
C. fascicularis with a row of dense tufts of brittle glassy 
spicule ; and lastly, in the great C. Si¢kensis, in which the 
mantle entirely envelopes the shell, the surface is crowded 
with very close-set minute stars of glassy spiculz. 
The imbedded portion of each plate or valve in the shell 
of Chiton is more or less produced posteriorly in two pro- 
cesses according to the oblong tendency of its structure, 
and the greater power necessary to sustain them in com- 
parative order; in most species where the valves are con- 
siderably narrower transversely than longitudinally, these 
posterior processes, termed apophyses, from their analogy 
in action to the apophysis in the osteology of the vertebrate 
skeleton, are but slightly developed, and the sinus between 
them is finely irregularly serrated. In valves of a more 
oblong form, as in C. incisus the apophyses become more 
prominent, until in Chitonellus where the valves are longer 
longitudinally than transversely, and isolated from each 
other, the apophyses are produced to an extent which 
enables them to sustain the valves, each by itself, within 
the mantle without the superposition of any cartilaginous 
expansion. 
The exposed portion of each valve in the shell of Chiton, 
and that only which is characterized by any sculpture or 
design of colouring, has the appearance of a convex shield, 
supported on either side by a raised triangular wing-like 
growth, forming as it were the radii of a circular plate 
which meet together in the anterior terminal valve; so 
that the sculpture and colouring of the cephalic valve is 
almost invariably the same as in the lateral areas of the 
rest; and the design of the central areas is as invariably 
different. 
The Chitons live attached to stones and fragments of 
shells in deep water, sometimes on exposed rocks, but most 
frequently under stones at about low-water mark. They 
exist in abundance on the south west shores of America, 
of Australia and New Zealand, the Eastern Archipelago, 
the Pacific Islands, Cape of Good Hope, West Indies, 
Sitka, and the shores of Europe and Asia. 
The genus Chiton has been subdivided by the Rev. 
Lansdowne Guilding, into the genera Acanthopleura and 
Kapellopleura, distinguishing the spiny and hairy species, 
into the genera Acanthochetes and Amicula, by My. Gray, 
to distinguish the fasciculate species, and those in which 
the mantle is extended over the shell, and a subgenus, 
under the name of Helminthochiton, has been proposed 
by Mr. Salter for the reception of the elongated species ; 
all these, however, can only be estimated as sections of the 
genus Chiton, and of a rank much inferior to that of Chiton- 
ellus, to which genus I refer for further observations. 
Species 1. (Mus. Cuming.) 
Curton Barnesit. Chit. testd subabbreviato-ovata, valvis 
angustis, terminalibus radiatim granosis, granis solita- 
riis, subirregularibus, valvis ceteris areis centralibus ad 
umbones levibus, deinde striis decussatis, quarum longi- 
tudinalibus fortioribus, areis lateralibus radiatim stria- 
tis, striis irregulariter et subrudé granosis ; valvis ter- 
minalibus ceterarum areisque lateralibus olivaceo-fuscis, 
centralibus castaneo-fuscis, umbonibus nigricantibus, 
maculé luteo-albidé utrinque subdistanter notatis ; 
ligamento latiusculo, fortiter granoso-coriaceo, viridi- 
olivaceo. ' 
