254 AN INTRODUCTION TO THE STUDY OF FOSSILS 



which enters the successive chambers through apertures in the 

 septa. 



The unoccupied chambers are filled by a gas which resembles 

 air but with a somewhat larger proportion of nitrogen. These 

 gas-filled chambers seem to support the shell in the water to a 

 sufficient extent to relieve the animal of its weight in swimming 

 or floating. 



The tubular prolongation of the mantle, i.e. the siphon left 

 behind in the apertures of the successive septa, secretes about 

 itself a calcareous wall, the siphuncle (Fig. 112, si.), just as the 

 posterior portion of the mantle elsewhere has built the septum. 

 That portion of the siphuncle, called the siphonal collar, project- 

 ing backwards a short distance from each septum, is built by the 

 enlarged anterior end of the siphon where it joins the mantle at 

 the body ; it is heavier and hence more likely to be preserved fossil 

 than the remainder of the siphuncle. 



An external depression or opening, called the umbilicus, is 

 visible at the center of the coils in Nautilus umhilicatus ; in 

 N. pompilius the outermost volution so envelops the inner ones 

 that no opening is visible. The dark portion of the coil just 

 inside the aperture is covered during life by a convex fold of the 

 mantle extending upward from the umbilicus of the shell. 



The body is held in the shell by strong muscles whose large 

 crescentic areas of attachment are clearly visible on the inside 

 of the living chamber near the umbilicus. The banded muscle 

 scar connecting these two areas marks the place of attachment 

 of three muscular (aponeurotic) bands which serve likewise in 

 attaching the body to the shell and are called collectively the 

 annular muscle or annulus (Fig. 112, ajt.ni.). 



The body when removed from the shell is found to be roughly 

 oblong in outline and between six and seven inches in length. 

 It is divided into a large, distinct head, bearing eyes and tentacles, 

 and a rounded trunk. The ventral portion of the body is on 

 the external or convex side of the shell, the dorsal on the inner 

 or concave side. 



