EVOLUTION OF FOSSIL CEPHALOPODA. 241 
isin 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. Zxdoceras, 
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. 1), 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 zzvolution) 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 Mautilus, 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 Mautilus. 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 Arietide, and Phylogeny of an Acquired Char- 
acteristic. 
