LIFE IN PRIMARY PERIOD 225 



or else folded in such a way that the line of junction of the 

 septum and the shell forms a line broken only at its 

 point of origin, 1 or a wavy line whose curves increase in 

 number and consequently decrease in size — the lines traced 

 thus becoming more and more complicated as the Jurassic 

 period advances ; these complicated lines of junction are 

 characteristic of Ammonites. The septa are traversed through- 

 out by a tube, the siphuncle, which among the still existing 

 Nautilidae, congregating in large numbers below the surface 

 of our warm seas, becomes attached to the top of the shell 

 which is perforated there by a slit ; whereas in the inner shell of 

 the Spirulidae, partitioned like that of the Ammonites but with 

 smooth walls like those of the Nautilidae, the siphuncle goes 

 through the last partition and terminates in a small ovoid 

 sac, the ovisac, which is attached to the top of the shell by 

 a ligament, the prosiphon. 



From the head of the Nautilus spread a number of discs, 

 bearing very mobile vermiform tentacles that give these 

 organisms a most characteristic appearance ; they have four 

 branchiae. The head of Spirula, on the contrary, is formed 

 like that of the Calamary and the Cuttle fish ; and has only 

 two branchiae. Munier-Chalmas, who discovered these 

 differences between the terminations of the siphons among 

 these creatures, has shown that Spirula resembles the 

 Ammonites in this respect. From this he concluded that the 

 latter, like the Spirulae, were dibranchiate Molluscs with ten 

 tentacles to their heads, whereas the large Cephalopods 

 with smooth septa should be grouped with the Nautilus. 



The Cephalopods, with straight and partitioned shells, 

 of Primary times have, however, a much larger siphuncle than 

 the true Nautilidae (Orthoceras). It is sometimes lateral 

 (Cyrtoceras), sometimes central, and the septa themselves 

 may be lateral (A scoceras) . We must conclude, therefore, 

 that the siphuncle was at first an integral part of the body, and 

 that in Orthoceres, whose straight shell can exceed two 

 metres in length, it is nothing but the integument of the 

 upper portion of the body, originally fixed to the shell but de- 

 tached on account of the weight of the latter, since the animal, 

 as has been said, swims with its ventral side uppermost. 



1 Goniatites, with siphon near the convex wall of the shell. Clymenia 

 with siphon on the opposite wall. 



Q 



