The shells of all the Tetrabranchiata agree in the following points:
1. The shell is external.
3. The outermost chamber of the shell is the largest, and is the one inhabited by the animal.
4. The various chambers of the shell are traversed by a tube, termed the "siphuncle."
Agreeing in all these fundamental points of structure, two very distinct types of shell may be distinguished as characteristic of the two families Nautilidae and Ammonitidae, into which the order Tetrabranchiata is divided.
In the family Nautilidae (fig. 234, d and e), the "septa" of the shell are simple, curved, or slightly lobed; the "sutures " are more or less completely plain; and the "siphuncle " is central, sub-central, or internal (i. e., on the concave side of the curved shells).
Fig. 234. - Diagram to illustrate the position of the siphuncle and the form of the septa in various Tetrabranchiate Cephalopoda. The upper row of figures represents transverse sections of the shells, the lower row represents the edges of the septa. a a Ammonite or Baculite ; b b Ceratite ; c c Goniatite ; d d Clymenia ; e e Nautilus or Orthoceras.
In the family Ammonitidae (fig. 234, a, b, and c), on the other hand, the septa are folded and complex; the sutures are angulated, zigzag, lobed, or foliaceous; and the siphuncle is external (i.e., on the convex side of the curved shells).*
* In the Ammonitidae, the initial chamber ("ovisac") of the shell is an egg-shaped chamber isolated from the first air-chamber by a distinct constriction, whereas no such arrangement obtains in the Nautilidae. Such a structure, however, is found in Spirula, Belemnites, and other allied forms ; and it has recently been concluded (Munier-Chalmas) that the Ammonitidae are properly Dibranchiate, their shell being an internal skeleton or phragmacone, similar to the shell of the Spirula.
In both these great types of shell, a series of representative forms exists, resembling each other in the manner in which the shell is folded or coiled, but differing in their fundamental structure. All these different forms may be looked upon as produced by the modification of a greatly elongated cone, the structure of which may be in conformity with the type either of the Nautilidoe or of the Ammonitidoe. The following table (after Woodward) exhibits the representative forms in the two families:
„ bent on itself, . . .
Ascoceras, . .
" bent on itself ...............
" curved, ........................
" spiral, ..........................
Gyroceras, . .
„ discoidal and produced,
Lituites,. . .
" involute, ..................
Nautilus, . .
After the Nautilus itself, the most important form of the Nautilidoe is the Orthoceras (fig. 235). In structure this was doubtless essentially identical with the Nautilus, but the shell, instead of being coiled into a spiral lying in one plane, was extended in a straight, or nearly straight, line. Orthoceratites of more than six feet in length have been discovered, but in all, the body-chamber, in which the animal was lodged, appears to have been comparatively small. The siphuncle is usually very complex in structure, and was calcareous throughout its entire length.
Fig. 235. - Orthoceras explorator, Billings. 1. Side view of a fragment, showing the septa. 2. Transverse section of the same, showing (s) the siphuncle.
The structure of the shell in the Ammonitidae is exactly that of the Pearly Nautilus, consisting of an outer porcellanous and an inner nacreous layer. The body-chamber was rather elongated than laterally expanded or dilated. The simplest form of the Ammonitidae is the Baculite, in which the shell is straight, like that of an Orthoceras, while the septa have the characters of those of an Ammonite, and the siphuncle is external. In the Turrilite (fig. 237) the structure of the shell is the same, but it is coiled into a turreted spiral. In the Ammonite itself (fig. 236), the shell is discoidal and involuted, corresponding (in form) to the shell of the Nautilus; the body-chamber was of comparatively large size, and had its aperture closed, in some species at any rate, by an operculum. The shell sometimes attained a gigantic size, and several hundred species of the genus have been described. In Crioceras (fig. 237) the shell was a flat spiral, like that of the Ammonites, but the whorls are not in contact. In Toxoceras the shell is shaped like a bow. In Ancyloceras (fig. 237) the shell is at first discoidal, with separate whorls, then produced into a straight line, and finally bent forwards into a hook.
Fig. 236. - Ammonites bifrons, from the Lias.