MORPHOLOGICAL DIFFERENTIATION. 343 



difference in the hard shell is seen in the curved and lobed 

 suture of the Goniatites as contrasted with the simple suture 

 of the Nautiloids. 



The law is again seen in force in the evolution of the Am- 

 monites, beginning in the Sicily and India Permian beds; by 

 the early part of the Trias this new type had expressed a 

 "Wonderful expansion. Out of the 92 genera described and 

 recognized by Zittel, 45 occur in the Triassic, representing 9 

 out of the 13 known families. 



Again, in the Jurassic the great differentiation of type 

 expressed in the Dibranchiates took place, not in a single form, 

 but both decapod and octopod modifications appear together. 



Thus we find this distinguishing character of the Dibran- 

 chiate (the consolidation of the siphonal tube, after the tube 

 with disunited edges had existed from Ordovician time 

 throughout the Paleozoic) making its first appearance at the 

 beginning of the Mesozoic, but thereafter continuing on in 

 successive and various forms until the present time. 



In each of these cases, of the initiation of new types of 

 the Cephalopod mode of organization, there was a rapid 

 evolution of the chief modifications of the new type near the 

 period of its first initiation among the geological faunas of the 

 world. 



