/» MANUAL OF THE MOLLUSCA. 



of wliich several specimens liave been brouglit 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 . trn'riUtcs. 



discoidal in gyroceras . crioceras. 



discoidal and pvduced in . lituitcs . . ancyloceras. 



mvolute in nautilus . , anunonites. 



Internally, the shell is divided into cells or chambers, by a series of parti- 

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

 by the animal, the rest are emptj^ during life, but in fossil specimens they are 

 often iiUed with spar. When the outer shell is removed (as often happens to 

 fossils,) the edges of the sej^ta are seen (as in PL III., figs. 1, 2.) Sometimes 

 they form cm'ved lines, as in nautilus and ortJioceras, or they are zig-zag, as 

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



Fig. 34. Suture of an ammonite.^ 



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

 elevations are called saddles, and the intervening depressions loies. 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, mth a very 

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

 fmmel -shaped, or bead-like tubes. In some of the oldest fossil genera, adi- 

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



* 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. Ben- 

 nett; it is drawn as if lying in the section of a shell, without concealing any part oi 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. heteroplnjllus, Sby., from the lias, Lyme Regis. British Museum. Only one 

 side is represented ; the arrow indicates the dorsal saddle. 



X From their resemblance to the sutures of the skull. 



