MOLLUSCA — CEPHALOPODA. 



151 



that the sutures are simple ; there are backwardly projecting Gallery 

 septal necks ; the outside of the shell is smooth or slightly rpa^e'cas 

 ribbed. For these, and other reasons, the cephalopods of i" 

 which we have been speaking are divided into two Orders : 

 the Ammonoidea and the Nautiloidea. 



Eeturning 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. ponipilius, and may be compared directly with the 

 similar internal cast of [b) the ammonite, s, suture ; m, mark left by 

 muscle of attachment ; I, lines 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 v^ere 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. Aulacoceras. In these the guard is short, and the cham- 

 bered shell-cone, when found without it, might be taken for 

 an Orthoceras ; but it must be noted that the siphuncle is at 

 the margin as in Ammonoidea. It is probable that, like 

 their living descendants, these animals lived an active life 

 and frequently 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 



