78 



MANUAL OP THE MOLLT7SCA. 





of which several specimens have been brought to Europe within the last few 

 years.* 



The shell of the tetrabranchiate cephalopods is an extremely elongated 

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



It is straight in orthoceras . baculites. 



bent on itself in ascoceras . ptychoceras. 



curved in cyrtoceras . toxoceras. 



spiral in trochoceras . turrilites. 



discoidal in gyroceras . crioceras. 



discoidal and produced in . lituites . . ancyloceras. 



involute in nautilus . 9 ammonites. 



Internally, the shell 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 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 orthoceras, or they are zig-zag, as 

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



Fig. 34. Suture of an ammonite.^ 



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

 elevations are called saddles, and the intervening depressions lobes. 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 nearly flat in the middle, and folded round the edge (like a shirt - 

 frill), 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 oi 

 funnel shaped, or bead-like tubes. In some of the oldest fossil genera, act I" 

 noceras, gyroceras, and phragmoceras, the siphuncle is large, and contains in 



* The frontispiece, copied from Professor Osven's Memoir, represents the animal oi 

 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 Museum, in which the relation of the animal to its shell is accurately shown. 



t A. heterophyllus, Sby., from the lias, Lyme Regis. British Museum. Only 

 side is represented ; the arrow indicates the dorsal saddle. 



J From their resemblance to the sutures of the skull, 



,. 



