180 MANUAL OF THE MOLLUSCA. 



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

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

 pressions lobes. 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 shii't -frill), 

 where they abut against the outer shell- wall (Fig. 44). 



The siphunde 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 

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

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

 radiating plates, like the lamellae 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 nautilidce 

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



Fig. 42, Nautilus. Fig. 43. Cljinenia. Fig. 44. Hamites.t 



The air-chambers 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 body-chamber 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 Pompilius, L. Fig. 43, Clymenia striata, Munst., see PI. II., 

 Fig. 16. Fig. 44. Haviites 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 tlie 

 eiphuncle. Mr. Jones, Wai'den of Clun Hospital, has several of these in apposition. 



