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 

 length. 



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. 



Order I. Dibranchiata. 



Animal with two branchije ; not more than eight or ten arms, 

 provided with suckers ; an ink-bag ; shell commonly internal and 

 rudimentary ; rarely external, but not chambered. 

 Section A. Octopoda. 



Arms dght, suckers sessile. 



