78 



MANUAL OF THE MOLLUSCA. 



of wMcli several specimens have been brought to Europe wdtliin the last few 

 yeai's.* 



The shell of the tetrabranchiate cephalopods is an extremely elongated 

 cone, and is either straight, or variously folded, or coUed. 



It is straight in orthoceras . baculites. 



lent on itself in ascoceras . ptychoceras. 



curved in cyrtoceras . toxoceras. 



spiral in trochoceras . tiurilites. 



discoidal in gyroceras . crioceras. 



discoidal and produced in . lituites . . ancyloceras. 



involute in nautilus . , ammonites. 



InteraaUy, the sheU is divided into cells or chambers, by a series of parti- 

 tions {septa), connected by a tube or siphuncle. The last chamber is occupied 

 by the animal, the rest are empty dm-ing life, but in fossil specimens they are 

 often filled wth spar. When the outer shell is removed (as often happens to 

 fossils,) the edges of the septa are seen (as in PI. III., figs. 1, 2.) Sometimes- 

 they form curved Hues, as in nautilus and orthoceras, or they are zig'Zag, as 

 in goniatites (fig. 53,) m foliaceous, as in the ammonite, fig. 34. 



?^tiiH%« 



Fig. 34. Suture of an ammonite A 



The outlines of the septa are termed sutures ;% when they are folded the 

 elevations are called saddles, and the intervening depressions lohes. In 

 ceratites (fig. 54) the saddles are round, the lobes dentated; in ammonites 

 both lobes and saddles are extremely complicated. Broken fossils show that 

 the septa are neai'ly flat in the middle, and folded round the edge (Hke a shirt- 

 fiiU), where they abut against the outer shell- wall (fig. 37). 



The siphuncle of the recent nautilus is a membranous tube, ■with a very ^ 

 thin nacreous investment : in most of the fossils it consists of a succession of,, 

 fuunel- shaped, or bead-like tubes. In some of the oldest fossil genera, acti->t 

 noceras, gyroceras, and phragmoceras, the siphimcle is lai'ge, and contains in. 



* TKe 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. Ben- 

 nett; it is drawn as if lying in the section of a shell, without concealing any part of it. 

 The -woodcut, fig. 43, is taken from a more perfect specimen, lately acquired by the- 

 British IMuseum, in which the relation of the animal to its shell is accurately shoAvn 



t ^. /?e<e/-ojj/;2/i^?/s, Sby., from the lias, Lj-rae Regis. British Museum. Only one • 

 side is represented ; the arrow indicates the dorsal saddle. J 



% From their resemblance to the sutures of the skull. 



1 



