EVOLUTION OF FOSSIL CEPHALOPODA. 245 
of the Jura and Cretaceous. In the Jura these ammon- 
ites reached their acme, branching out into very many 
families and subfamilies, increasing usually in complex- 
ity of sutures and variety of ornamentation. In the 
Cretaceous they gradually declined, dropping off one at 
a time until all were gone. The total number of Am- 
monoidea now described reaches about five thousand, of 
which only a few hundred belong to the Paleozoic gonia- 
tites, the others belonging to the ammonites of the Car- 
boniferous, Permian, and Mesozoic. Later than this no 
ammonoids are known. 
Only simple radicles or stocks persist, but from time 
to time certain genera branch off from the main stock, 
become highly specialized, and often give rise to so- 
called abnormal* forms, phylogerontic or degenerate 
genera (retrogressive), which do not perpetuate their 
race. These leave their close coil, becoming straight, 
as Baculites (Plate V, Fig. 13); ascending spiral, as Tur- 
rilites (Plate V, Fig. 12) ; hook-shaped, as Macroscaphites 
(Plate V, Fig. 14); or open-coiled, gyroceran, as Crio- 
ceras (Plate V, Fig. 11). These do not form a natural 
group, but are themselves even in some cases polyphy- 
letic, as shown by their ontogeny; their larval stages, 
however, as shown even by the straight Baculites (Plate 
V, Fig. 13), all correspond to various normal close- 
coiled progressive genera, such as Lytoceras (Plate V, 
Fig. 10). 
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 dis- 
appeared as suddenly ; Afed/icottia reached its culmina- 
tion in the Permian, barely managed to live on until the 
Trias, and disappeared without posterity, while the main 
* J. F. Pompeckj. ' Ueber Ammonoideen mit Anormaler Wohn- 
kammer. Stuttgart, 1894. 
