148 GUIDK TO THE FOSSIL INVEnTKBUATE ANIMAIiS. 
Gallery 
VII. 
Table-case 
I. 
lay the much constricted visceral cone. H.vamination of 
polished sections across Endoccras and allied forms shows 
within this septal neck-tube yet another series of structures, 
called sheaths, and somewhat like a ]>ile of funnels stuck one 
inside the other (Kigs. 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 
^ — a/ierture of shell 
■ hody-chamher \ 
jiartly filed by f 
• matrix ' 
■ shell-ieall 
sejdum 
se2>tal neck 
f siphiiitele 
suture, seen between 
matrix filHiig cham- 
bers 
ornamented outside of 
shell-wall 
apex 
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 {to) of the 
wide neck-tube ; sc, space occupied by visceral cone, the hardened 
skin of which forms the sheath (s/t); r, remains of similar sheaths; 
c, endosiphon. (After Poord). 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 — tlie sipluuicle. 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 tliero is an advance on 
