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1846..
SALTER ON FOSSIL CHITONS.
49
accompanied by differences in the structure of tlie mantle and the
thickness or expansion of the entire form.
Lamarck had long ago described a tertiary species, C. grignonensiSy.
figured by Deshayes ; and Cantraine, in his * Malacologie Mediterr.
et littorale,' added another, C. subapenninus, from Italy. As both
these belong to the common form of Chiton, no more need be said of
them. Count Miinster in his ' Beitrage' first described a species
from the carboniferous strata of Tournay, under the name of Chiton
priscus, and from detached valves, which are very numerous there,
he reconstructed the shell (fig. 2). Dr. Sandberger added two species
from the Devonian rocks of Vilmar, C.fasciatus and C. subgranosus,
and subsequently De Koninck figured two more new ones, C. gem-
matus and C. concentricus^ adding a third, C cordifer^ which however
by his own consent is now admitted to be an Encrinital plate. These
eight species were all that were known fossil, until very lately the
Baron de Ryckholt described ten additional ones from the carboni-
ferous and Devonian rocks of Tournay and Vise. The notice of
these appears in an elaborate paper on the external structure of the
shell, in which rules are given for reconstructing the entire shell,
when only one or part of one cerame or plate is discovered*.
Fig.l
Fig. 2
Fig. 4.
Fig. 1. Chiton Cumingii.
2. Helminthochiton priscus.
3. H. eburonicus.
Fig. 4. Chitonellus.
5. Chiton amiculatus.
6- HelmJnthochiton Griffithii.
With two exceptions, all the Devonian and carboniferous species
(figs. 2, 3) much resemble each other, and their relations to living
forms seem only to be with such as C. incisus and C. alatus of the Phi-
lippines, in which the plates are lengthened, and their contour square
instead of transversely oblong. The ordinary form of Chiton is seen
in fig. 1, C. Cumingii, a species from Valparaiso, and may be re-
cognised in the common C. cinereus of our own shores ; the plates
are transverse, and the sides are marked by a diagonal line, fig. 1 a,
the ornaments behind which are of a different character to those in
front of it. The two areas .d and e) are called respectively the
dorsal and lateral areas. The under surface is marked for the attach-
ment of muscles, and is imbedded in the surface chiefly by an ex-

• Bulletin de I'Acad. Royale de Bruxelles, torn. xii. 2nde partie, 1845, p. 36.