CEPHALOTODA. 



179 



It was long ago remarked by Dillwyn, that shells of the car- 

 nivorous gasteropods "were almost, or altogether, wanting in tho 

 palaeozoic and secondary strata; and that the office of these 

 animals appeared to have been performed, in the ancient seas, 

 by an order of cephalopods, now nearly extinct. Above 2,000 

 fossil species belonging to this order are now known by their 

 shells ; whilst their only living representatives are a few species 

 of nautili.* 



The shell of the tetrabranchiate cephalopods is an extremely 

 elongated cone, and is either straight, or variously folded, or 

 coiled. 



It is straight in . 

 hent on itself in 

 curved in 

 spiral in 

 discoidal in . 

 discoidal and produced in 

 involute in . 



orthoceras 



ascoceras 



cyrtoceras 



trochoceras 



gyi'oceras 



lituites 



nautilus 



baculites. 



ptychoceras. 



toxoceras. 



turrilites. 



crioceras. 



ancyloceras. 



ammonites. 



Internally, the shell is divided into cells or chambers, by a 

 scries of partitions [septa], connected by a tube or siphuncle. The 

 last chamber only is occupied by the animal. The others are 



Fig. 41. Sutui-e of an ammonite.t 



probably occupied in succession. They are empty during life, 

 but in fossil specimens they are often filled with spar. When 

 the outer shell is removed (as often happens to fossils), the edges 

 of the septa are seen (as in PL III., Figs, 1, 2). Sometimes they 

 form curved lines, as in nautilus and ortJioceras, or they aro 

 zigzag, as in goniatites (Fig. 60), ov foliaceous, as in the ammonite 

 (Fig. 41). 



* The frontispiece, copied from Professor Owen's Memou", represents the animal of 

 the first nautilus, captured off the New Hebrides, and brought to England by Mr. 

 Bennett ; it is drawn as if Ij'ing in the section of a shell, without concealing any part 

 of it. The woodcut. Fig. 50, is taken from a more perfect specimen, subsequently 

 acquired by the British Museum, in which the relation of the animal to its shell is 

 accurately shown. 



t A. heterophylbis, Sby., from the lias, Lyme Eegis. British Museum. Only on« 

 side is represeiited ; the aiTow indicates the dorsal saddle. 



