148 GUIDE TO THE FOSSIL INVEKTEBRATE ANIMALS. 



Gallery 

 VII. 

 Table-case 

 1. 



lay the much constricted visceral cone. Examination of 

 polished sections across Eiidoceixis and allied forms shows 

 within this septal neck-tube yet another series of structures, 

 called sheaths, and somewhat like a pile of funnels stuck one 

 inside the other (Figs. 79 c, 80 a). These indicate that, as the 

 animal advanced in its shell, its viscera naturally went with 

 it, and towards the void thus left the walls of the visceral 

 cone were still further sucked in. Thus there tended to 



■ aperture of shell 



■ hody-chamher partly 

 filled by matrix 



' shell-wall 



septum 

 septal neck 



siphuncle 



suture, seen between 

 matrix filling cham- 

 bers 



ornamented outside of 

 shell-wall 



Fig. 80. — Primitive Nautiloidea. a, diagram of a section through the 

 middle of Piloceras, the dotted lines being reconstructed ; sw, shell-wall ; 

 s, chambers divided by septa, whose necks form the wall (w) of the 

 wide neck-tube ; sc, s^Dace occupied by visceral cone, the hardened 

 skin of which forms the sheath (sh) ; r, remains of similar sheaths ; 

 e, endosiphon. (After Foord). b, Orthoceras ; in the lower part the 

 shell-wall is preserved ; then it is partly removed, showing the filling of 

 rock or matrix ; higher up this is cut through, and at the top it is 

 removed, showing the other side of the shell. 



arise a narrow and empty tube — the siphuncle. The walls, 

 however, were stiffened with lime and did not completely 

 yield to the suction, so that, when the animal again advanced, 

 the inner layers of the skin were torn away from the outer 

 ones. These inner layers thickened and stiffened in their 

 turn, and the process was repeated. Thus arose the thin 

 tube, sometimes called the endosiphon, and a series of sheaths 

 attached to it. 



In Orthoceras and similar forms there is an advance on 



