MOLLUSCA — CEPHALOPODA. 
151 
that the sutures are simple ; there are backwardly projecting 
septal necks ; the outside of the shell is smooth or slightly 
ribbed. For these, and other reasons, the cephalopods of 
.which we have been speaking are divided into two Orders : 
the Ammonoidea and the Nautiloidea. 
Keturning now to the straight shells, we shall note that 
none of those without a protoconch survive the Palaeozoic 
Era, but that all give place to coiled Nautiloidea. Those 
with a protoconch, however, do not all give place to 
Ammonoidea, but some of them begin another line of 
evolution. In these the chambered shell becomes shorter, 
and it is believed that folds of the mantle were turned back 
right over the shell to its very apex, thus affording a 
Fig. 82. — A nautilus and an ammonite, a is a plaster cast of the inside of 
the shell of N. pompilius, and may bo compared directly with the 
similar internal cast of (b) the ammonite, s, suture ; w, mark left by 
muscle of attachment; I, linos left by this mark in its previous 
positions. Less than natural size. 
protection to the shell and to its protoconch. These 
mantle-folds continued their activity in secreting shell- 
substance, and so there were deposited outside the apex of 
the shell a number of layers, forming a solid guard (Fig. 85 a). 
Such forms first appear with certainty in the Trias, 
e.g. Aulacoccms. In these the guard is short, and the cham- 
bered shell-cone, when found without it, might be taken for 
an Orthocera-a ; but it must be noted that the siphuncle is at 
the margin as in Ammonoidea. It is probable that, like 
their living de.scendants, these animals lived an active life 
and fre(iuently shot backwards by discharge of water from 
the funnel; thus the value of the guard is obvious. The 
further history of these forms shows the rise of various 
Gallery 
VII. 
Table-ease 
1 . 
