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 shirt-frill), 
where they abut against the outer shell-wall (Fig. 44). 
The stphuncle 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 lamelle of a coral. The position of the 
siphuncle is yery variable; in the ammonitide it is external, or 
close to the outer margin of the shell (Fig. 44) In the nautilidee 
it is usually central (Fig. 42), or internal (Fig. 43). 
Fig. 42, Nautilus. Fig. 43. Clymenia. Fig. 44. Hamites.} 
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.} 
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. 
+ Fig. 42. Nautilus Pompilius,L. Fig. 43. Clymenia striata, Miinst., see Pl. II-, 
Fig. 16. Fig. 44. Hamites cylindraceus, Detr., see Fig. 65. 
+ 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 
siphurcle, Mr. Jones, Warden of Clun Hospital, has several of these in apposition. 
