The ‘tentacles’ are about 90 in number, and consist of four groups each of 12 or 13 labial tentacles surrounding the mouth, two groups each of 17 larger (brachial) tentacles on each side of the head, two thicker tentacles which combine to form the ‘hood,’ and two small tentacles on each side of the eye. When the animal swims, the tentacles are extended radially from the head, somewhat like those of a sea-anemone. The direction of the many pairs of tentacles at constant but different angles from the head, is the most striking feature in the living Nautilus, and accounts for its being described, when seen on the surface, as ‘a shell with something like a cauliflower sticking out of it.’[402] The funnel is not a complete tube, but is formed by the overlapping of the margins of two thin fleshy lobes (which are probably morphologically epipodia), so that when the two lobes are parted, a broad canal appears, leading to the branchial cavity. The head is conical, and the mouth and its appendages can be retracted into a sort of sheath, over which fits the ‘hood.’
Fig. 255.—A, Gomphoceras ellipticum M’Coy, Silurian: B, aperture (ap) of same; s, s, septa; si, position of siphuncle. (After Blake.)
Other genera are Trocholites, Gyroceras, Hercoceras, Discites, Aturia.—Ordovician to present time.
Fam. 11. Bactritidae.—Shell straight, conical, siphuncle small, marginal, septal necks long, funnel-shaped, sutures undulating, with a sinus corresponding to the siphuncle. This family, from the form of its sutures, appears to constitute a passage to the Ammonoidea. Single genus, Bactrites.—Silurian and Devonian.
(b) Prosiphonata.—Septal necks directed forwards.
The two genera are Bathmoceras (Ordovician), shell straight, conical always truncated, siphon marginal; and Nothoceras (Silurian), shell nautiloid with simple sutures.
Sub-order 2. Ammonoidea.—Shell multiform, straight, curved, flat spiral, or turreted, sutural line more or less complex, siphuncle simple.
Some authorities hold that the members of this great sub-order, now totally extinct, belong to the Dibranchiata, on the ground that the protoconch resembles that of Spirula rather than that of the Nautiloidea. Others again regard the Ammonoidea as a third, and distinct Order of Cephalopoda. Their distribution extends from the Silurian to (possibly) the early Tertiary. No trace has ever been found of an ink-sac, mandible, or hooks on the arms; the shell was undoubtedly external.
