With the exception of Goniatites, the Ammonitidce are peculiar to, and co-extensive with, the secondary strata ; while the Ha:Whiles, with the exception of Nautilus and Aturia, are confined to the paheozoic rocks. But the paheozoic so-called Nautilielce exhibit peculiarities suggesting very wide differences from the modern pearly Nautilus. It has been proposed to associate the greater part of them with the Orthocerata as a distinct family, but at present the data are defective. Like the Ammaniticke, their shells assume almost every conceivable form and curvature, and the genera founded on these charac ters are very ill defined.
The simplest form of Orthoceras is like a Nautilus unrolled ; and Lituites (fig. 20, 2) is the same with the apex spiral. Some of the carboniferous Nautili have a square back, and the whirls either compact or open in the centre (fig. 20, I) ; whilst the last chamber is more or less disunited. The species with the whirls quite disunited constitute the genus Trigonoceras, M`C. (= Nautiloceras, d'Orb.) The Silurian genus Trochoceras, Barr, is a spiral Nautilus. Clymenia, a characteristic Devonian fossil, has angular sutures and an internal siphuncle ; it may perhaps be coiled up ventrally like the Spirula. The tertiary shell called Nautilus zia-sac (Aturia, Br., fig. 19, 2, z), which is so widely distributed in Europe, America, and India, has a siphuncle nearly marginal when young, but gradually becom ing more central in the adult : it has no special relation to Clymenia.
Those species of Orthocerata in which the aperture is con tracted form the genus Apioceras, Fischer ( = Poteriecera-s, M`C.), or when also curved, the Oncoceras of Hall. In Bar rande's genus Ascoceras (fig. 20, the shell is flask-shaped, the chambered and siphunculated apex being apparently deciduous ; the aperture is contracted, and the air-chambers occupy only the dorsal half of the shell. In Phragmoceras (fig. 20, 7), the shell is slightly curved to the ventral side, and the aperture is remarkably contracted, the opening for the respiratory funnel being nearly distinct from the cephalic aperture. In Cyrtoceras the curvature is dorsaL In some other members of this family the siphuncle attains a remarkable size or extraordinary complexity.
name Discosorus to a fossil which is evidently the siphuncle of some very delicate and perishable chambered shell (fig. 20, 6). In those Orthocerata with siphuncles most nearly resembling the Discosorus they diminish rapidly towards the last chamber. Perhaps the most remarkable fossil of this group is the Huronia (fig. 20, s), found in the upper Silurian limestone of Drum mond Island. Siphuncles 6 feet in length and 14 inch in diameter, were seen by Dr. Bigsby in the cliffs ; they are sili cified, and stand out in bold relief from the matrix, but are un accompanied by any vestige of the shell, except in one or two instances, where the septa are faintly indicated by coloured lines. They are sometimes overgrown with coral, and were t. Omalii, de lion.; Carboniferous, Belgium.
5. Siphuncle of Huronia Bigsbyi, Stokes ; with outline of shell, and septa.
evidently so durable as to remain on the sea-bed long after the shell itself had decayed. The joints of the siphuncle are swollen at the upper part, and the interior is filled with an irregularly-radiated structure, apparently produced by the plaiting and calcification of the lining membrane. This struc tune also exists and is very regular in the siphuncle of the Devonian Orthoceras triganale, and in the shells referred to Oyroceras by d'Orbigny (fig. 20, s) ; also in Actinoceras, a sub genus of Orthoceras, discovered by Dr. Bigsby, and described by Stokes (Owl. Trans., vol 1., 1825). The plication of this in terior structure takes place in segments corresponding to the septa, and meeting in the centres of the siphuncular beads, leaving spaces or foramina for the passage of blood-vessels to the lining membrane of the air-chambers.* The vascularity of the latter is well shown in the impression of septa on the fine mudstones of the Ludlow rock, often mistaken for Spon garia, which they somewhat resemble.