PALAZAONTOGENY AND PHYLOGENY S17 
certain genera branch off from the main stock, become highly 
specialized, and often give rise to so-called abnormal’ forms, 
such as Hamutes, Baculites, Crioceras, Scaphites, phylogerontic or 
_ degenerate genera, which do not perpetuate their race. These 
do not form a natural group, but are themselves even in some 
cases polyphyletic, as shown by their ontogeny; so far as exam- 
ined their larval stages all correspond to various normal genera. 
Of course there were phylogerontic genera that were not 
abnormal in shape; thus Clymenia branched off in the Upper 
Devonian into a variety of species, and disappeared as suddenly; 
Medhicottia reached its culmination in the Permian, barely man- 
aged to live on until the Trias, and disappeared without pos- 
terity, while the main stock of unspecialized Prolecamtide 
endured as long as the race. The number of phylogerontic 
forms increases in the Mesozoic, showing a constantly increas- 
ing tendency to become abnormal, until before the end of the 
Cretaceous the entire race of ammonoids becomes phylogerontic, 
and dies out from sheer lack of plasticity to modify itself further 
with changing conditions. 
Such a general view or family tree of the ammonoids may 
be seen in any of the text-books of paleontology, especially 
those of Steinmann,? and of K. von Zittel,3 where we get the 
best attempts to represent our present knowledge and ideas of 
the genetic relationships of ammonites. These genealogies are, 
however, purely tentative, based not on ontogeny but on com- 
parison of series of adults. This would undoubtedly be the 
safest way if we had a perfect series of genera and species, but 
such a thing is unknown, and can never be obtained, on account 
of the incompleteness of the geologic record, and the mixing of 
faunas by migration in the past. 
The researches of Hyatt, Branco, and Karpinsky have given 
us a surer way; from their work we have learned that the 
Ammonoidea preserve in each individual a complete record of 
tJ. F. Pompecky, Ueber Ammonoideen mit Anormaler Wohnkammer. Stuttgart, 
1894. 
? Elemente der Palzontologie. 1890. 3 Grundziige der Palzeontologie. 1895. 
