40 THE LIFE OF THE MOLLUSCA 
Octopods there is no longer any true shell, but only 
some simple chitinous rudiments, to which the 
retractor muscles of the head and funnel are 
attached. In certain fossil Dibranchia (Belemnites, 
etc., Plate XXIII.) the internal chambered shell, 
known as the “phragmocone,” is enclosed in a 
pointed calcareous sheath, or “ guard,” at the end 
opposite to the head, while from the dorsal margin 
of the anterior end of the phragmacone there arises 
a broad, thin, chitinous plate, called the “ pro- 
ostracum.” To modifications in form of these three 
constituent parts, or to the partial or total suppres- 
sion of one or more of them, the resultant differences 
between the internal shells of the other members of 
the order may be traced. This, however, will be 
better gathered from the diagrammatic figures on 
Plate XXIV. than from any lengthy description. 
The shell of the Paper Nautilus, or Argonaut 
(Plate XXI., Fig. 4; Plate XXV., Fig. 12), stands 
on a different footing to the ordinary shell, and does 
not originate in the shell-gland; it is only formed 
some days after the creature is hatched, and is 
peculiar to the female, being chiefly used as a 
vehicle for carrying and protecting the eggs, which, 
when deposited, are agglutinated to it. It is exo- 
gastric, and composed of three layers, of which the 
outer and inner are alike and prismatic, while the 
middle one is fibrous: there is no pearly layer 
within. The animal is not attached to it in any 
