. A manual of zoology for the use of students : with a general introduction on the principles of zoology . Zoology. 334 MANUAL OF ZOOLOGY. doubtless essentially identical with the Nautilus, but the shell, instead of being coiled into a spiral lying in one plane, was ex- tended 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 Fig. 1
. A manual of zoology for the use of students : with a general introduction on the principles of zoology . Zoology. 334 MANUAL OF ZOOLOGY. doubtless essentially identical with the Nautilus, but the shell, instead of being coiled into a spiral lying in one plane, was ex- tended 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 Fig. 121.—Orihoceras eb:fihraior, BiUings. i. Side 'view of a fragment, showing the sep^a. z. Transverse section of the same, showing is) the siphuncle. The structure of the shell in the Ammonitidce 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 Ammonitidce is the Baculite, in which the shell is straight, like that of an Orthoceras, whilst the septa have the characters of those of an Ammonite, and the siphuncle is ex- ternal. In the Turrilite the structure of the shell is the same, but it is coiled into a turreted spiral. In the Ammonite itself, the shell is discoidal and involuted, corresponding (in form) to the shell of the Nautilus; the body-chamber was of compara- tively 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 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 the shell is at first discoidal, with separate whorls, then produced into a straight line, and finally bent forwards into a hook. Synopsis of the Families of the Cephalopoda. Class Cephalopoda. Ord
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