242 FOOT-NOTES TO EVOLUTION. 
then a gyroceran, before taking on its own characters; 
each gyroceran form went through these in the same 
order up to its adult characters, but never reached the 
close-coiled nautilian stage; while the orthoceran forms 
remained in that stage of development all their lives. 
These genera were all progressive, and are numerous 
branches radiating from a common stock or radicle, the 
primitive straight nautiloid. Thus different nautilian 
forms may resemble each other closely, and yet be 
actually more closely related to the radicle form than 
they are to each other. Formerly such species were 
grouped together in one genus, and called “representa- 
tive” species; now we know them to be merely morpho- 
logical equivalents in different lines of descent. 
When the close coiled stage was reached the nautilian 
shell had reached its limit, and could progress no fur- 
ther, and at once some of the stock began to retrograde. 
This is beautifully shown in the development of Lituztes 
(Plate V, Fig. 6), which goes through the orthoceran, 
cyrtoceran, gyroceran, and nautilian stages, and as it 
becomes adolescent leaves the close coil and reverts to 
the orthocerantype. A number of other nautilian genera 
acted in this way, giving rise to a number of aberrant 
types. These reversionary nautiloids are confined to 
the Paleozoic, and did not in any case become radicles of 
later groups; they had run their course, exhausted the 
possibilities of development, and died out without 
descendants. But the old simple orthoceran type held 
out until the Trias, and the unspecialized nautilian shell 
endured until the present time, although now rapidly 
nearing extinction. 
The animals that are capable of giving the best proof 
of evolution are the ammonoids. Somewhere back in 
the Silurian an Orthoceras by acceleration of develop- 
ment finally acquired a calcareous protoconch, or em- 
