EVOLUTION OF FOSSIL CEPHALOPODA. 



241 



is in doubt. The first nautiloid known occurs in Cam- 

 brian strata, and belongs to the straight-shelled Ortho- 

 ceras type, which is the radicle of the group. Endoceras, 

 the most primitive of orthoceran forms, prevails in the 

 Lower Silurian, but here come in also curved forms, at 

 first sparingly, then later abundantly. The simple un- 

 specialized orthoceran type (of which a member is fig- 

 ured on Plate V, Fig. i), survived throughout the entire 

 Paleozoic, and finally disappeared near the end of the 

 Trias. The first of the curved forms departed but little 

 from their ancestral habit, as shown on Plate V, Fig. 2 ; 

 but this is enough to give it a new generic title, Cyrto- 

 ceras. As time went on the curving became more pro- 

 nounced, as in the species shown on Plate V, Fig. 3, 

 still of the cyrtoceran type. Finally the coil became 

 complete, although the successive whorls did not touch 

 the preceding ones; this stage of evolution (although 

 in this case it is really involution} is called Gyroceras, a 

 species of which is figured on Plate V, Fig. 4. Later 

 still the successive coils began to touch and finally to 

 embrace the preceding, and the culmination of the nau- 

 tiloids is reached in Nautilus, as shown in Plate V, Fig. 

 5. These were formerly looked upon as generic stages, 

 but Professor Hyatt * has shown that there were many 

 straight forms that were not Orthoceras, many bent forms 

 besides Cyrtoceras, many loosely-coiled forms that were 

 not Gyroceras, and many close-coiled forms in addi- 

 tion to Nautilus. In other words, these correspond- 

 ing stages of development were merely morphological 

 equivalents of each other in different parallel lines of 

 descent from the remote straight-shelled parent stock. 

 Each nautilian form in its own development went 

 through first an orthoceran stage, then a cyrtoceran, 



* Genesis of the Arietidce, and Phylogeny of an Acquired Char- 

 acteristic. 



