Manual of zoology ; with a general introduction on the principles of zoology

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1885 Excerpt: ...the septa. 2. Transverse section of the same, showing (s) the siphuncle. The structure of the shell in the Ammonitidm 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 Ammonitida is the Bacuiile, 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 external. In the Turrilite the structure of the shell is the same, but it is coiled into a turreted spiral. In the Ammofiite itself, 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 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. Order I. Dibranchiata. Animal with two branchife; not more than eight or ten arrhs, provided with suckers; an ink-bag; shell commonly internal and rudimentary; rarely external, but not chambered. Section A. Octopoda. Arms eight, suckers sessile. Fam. I. Argonautida. Female provided with a calcareous, external, mjnothalamous shell, secreted by the webbed extremities of the dorsal arms. Gen. Argonauta. Fam. 2. Octopodidce. Shell internal, udiinentary, uncalcified. No pallia...