MOLLUSCA — CEPHALOPODA. 
157 
and the pliragmocono. Tliis la.st i.s relatively much wider 
than in the beleninite and is covered hy no solid guard, but 
by a thin coating of horny substance deposited by the 
mantle, which is still seen surrounding it (Fig. 85 /). 
The reduction of guard and phragmocone has progressed 
even further in Gcotcuthis hravipinnis. A specimen of this 
from the same rock preserves the 10 very short arms, 
mantle, and tail-tins, and shows a broad thin shell, such 
as in the squids and calamaries now living is called a 
pen, with very little trace of any chambered portion 
(Fig. 85 fj, h, i). A similar but narrower pen is found 
in ricsiofcuthis prisca, of which specimens from the Solen- 
hofen Stone are exhibited, one of them with the soft parts 
also indicated (compai-e Fig. 80 h). These pens should 
be compared with that of the squid, Loligo vulgaris, which is 
shown beside a glass model of the complete animal (Fig. 78 c). 
Other models of living cephalopods with similar shells are 
exhibited, but cannot here be discussed. It is enough to realise 
that in one line of descent of these forms with ensheathed 
shell the chambered cone, long protected by a stout covering, 
retained its calcareous and septate nature, while in another 
line it became horny, and ultimately lost its septa. 
There was however another line of evolution, the origin 
of which is difficult to trace, because one of its conspicuous 
characters was the complete loss of the shell. Another 
character was the absence of the long arms, reducing the 
number to eight. These forms therefore are called Octopoda, 
in o})position to the ten-armed squids and cuttle-fishes, 
which have been called Decapoda. Study of the early 
development of an octopus teaches us that its ancestors must 
have had a shell, and it seems probable that the loss was due 
to a reduction like that which took place in the squids, but 
Fig. 87. — Female Argonaut swimming from left to- right. 
greater in extent. A drawing of the oldest Octopod known 
is placed in this Case. Beside it are glass models of living 
species ol Octopoda, among which Argonauta deserves 
mention. As shown by the models and by the exhibited 
specimens, the well-known Argonaut .shell is of a different 
Gallery 
VII. 
Table-case 
1 . 
Between 
Wall-cases 
1 & 2 . 
