CEPHALOPODA. 179 
It was long ago remarked by Dillwyn, that shells of the car- 
niyorous gasteropods were almost, or altogether, wanting in tho 
paleeozoic 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. 
Itisstraightin . : . orthoceras .  baculites. 
bent on itselfin . . ascoceras . ptychoceras. 
curvedin 5 . cyrtoceras . toxoceras. 
Spirabin  . : . trochoceras . turrilites. 
discoidalin . : . gyroceras . crioceras. 
discoidal and producedin lituites . ancyloceras. 
involute in . ° nautilus . ammonites. 
Internally, the shell is Siies into cells or chambers, by a 
series of partitions (septa), connected by a tube or siphuncle. The 
last chamber only is occupied by the animal. The others are 
Fig. 41. Suture of an ammonite.} 
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. ITI., Figs, 1, 2). Sometimes they 
form curyed lines, as in nautilus and orthoceras, or they are 
zigzag, aS in goniatites (Fig. 60), or foliaceous, as in the ammonite 
(Fig. 41). 
* The frontispiece, copied from Professor Owen’s Memoir, represents the animal of 
the first nautilus, captured off the New Hebrides, and brought to England by Mr. 
Bennett; it is drawn as if lying 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. 
+ A. heterophyllus, Sby., from the lias, Lyme Regis. British Museum, Only ona 
side is represented ; the arrow indicates the dorsal saddle, 
