EVOLUTION OF FOSSIL CEPHALOPODA. 



245 



of the Jura and Cretaceous. In the Jura these ammon- 

 ites reached their acme, branching out into very many 

 famihes and subfamiUes, 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 Cfymenia branched off in 

 the Upper Devonian into a variety of species, and dis- 

 appeared as suddenly ; Medlicottia 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. 



