File:The Quarterly journal of the Geological Society of London (12981157933).jpg

376
PROCEEDINGS OF THE GEOLOGICAL SOCIETY. .DeC. 1,
PiLocEEAs *, gen. nov.
A broad, conical, and slightly curved shell, subcylindrical (or
compressed). Siphuncle and septa combined as a series of conical
concave septa, which fit into each other sheathwise.
PiLOCEEAS rsrvAGiNATUM, spcc. nov.
PI. XIII. figs. 17-21 ; and Woodcut, fig. 7, c.
P. 5 uncias longum, conicum, subcurvum, ore ovali, (long, oris ad lat. utll-8).
This singular shell, which seems to offer the simplest of aU Ce-
phalopodous forms (and to combine in its septal plates both septum
and siphuncle), is not unfrequent in the Durness limestone, in a
silicified state. None of the several specimens collected by Mr.
Peach are perfect.
The mouth is oval, with the longer diameter about 2 inches, and
the breadth 1^ inch, in specimens which would measure above 4
inches long. The general form is gently curved, ending in a some-
what obtuse point (fig. 19), near which the tube is annulated rather
strongly, while in the older portions it is more faintly so.
The septa, at a diameter of l^- inch, are rather more than one ith
of an inch apart, and viewed in section (fig. 21) show only three con-
centric sheaths. There is clear evidence only of four septa in the
longitudinal section (fig. 20), the remaining lines being due to the
crystallized linings of the chambers. These septa end in a sharp
point downwards, the walls meeting at an angle of about 35°, or less.
Comparison is naturally sought for this remarkable genus among
the numerous Cephalopods of the same zone. In America the Or-
thocerata with large lateral siphuncles (woodcut, fig. 7, a) abound,
Fig. 7. — Diagram, of some forms of Lower Silurian Orthoceratidce.
a. Cameroceras. h. Endoceras. c. Piloceras.
and in some of these the diameter of the siphuncle is so great in
proportion to the size of the shell, as almost to realize the form now
under consideration. Endoceras proteiforme, Hall, Pal. New York,
vol. i. pi. 48, &c., and still more E. magniventrum, pi. 53, show not
only a great siphuncular space, but also a pointed termination of the
siphuncle itself (fig. 7, h), which thus appears as an elongated lobe or
process of the hinder part of the body, hke one of the many lobes of

• From TTtXos, a cap, and the usual termination Kepas.