180 



MANTJAI, OF THE MOLLTISCA. 



The outlines of the septa are termed sutures ;* when they are 

 folded the elevations are called saddles, and the intervening de- 

 pressions loles. In ceratites (Fig. 61) 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. 44). 



The sipliuncle 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 funnel-shaped, or bead-like tubes. 

 In some of the oldest fossil genera, actinoceras, gyroceras, and 

 pliragmoceras, the siphuncle is large, and contains in its centre a 

 smaller tube, the space between the two being filled up with 

 radiating j)latos, like the lamellce of a coral. The position of the 

 siphuncle is very variable ; in the ammonitidce it is external, or 

 close to the outer margin of the shell (Fig. 44) In the nautilidoe 

 it is usually central (Fig. 42), or internal (Fig. 43). 



Fig. 42. Nautilus. 



Fig. 43. CljTiienia. 



Fig. 44. Hamites.t 



The air-chamhers of the recent nautilus are lined by a very 

 thin, living membrane; those of the fossil orthocerata retain 

 indications of a thick vascular lining, connected with the animal 

 by spaces between the beads of the siphuncle. J 



The hody-cJiamher is always very capacious ; in the recent 

 nautilus its cavity is twice as large as the whole series of air- 

 cells ; in the goniatite (Fig. 46) it occupies a whole whorl, and 

 has a considerable lateral extension ; and in ammonites communis 

 it occupies more than a whorl. 



The margin of the aperture is quite simple in the recent nautilus, 



* From their resemblance to the sutures of the skull. 



t Fig. 42. Nautilus Pompiliics, L. Fig. 43, CHymema striata, Miinst., see PI. II., 

 Fig. 16. Fig. 44. Hamites cylindraceus, Defr., see Fig. 65. 



X Most of the so-called spongaria are detached septa of an orthoceras, from the 

 Upper Ludlow rock, in which the vascular markings distinctly radiate from the 

 eiphur.ele. Mr. Jones, Warden of Clun Hospital, has several of these in apposition. 



