244 



FOOT-NOTES TO EVOLUTION. 



forms from the goniatites of the Paleozoic, to the ammon- 

 ites of the Mesozoic, was known long ago ; these natu- 

 ralists knew, too, that ammonites went through a gonia- 

 tite stage in youth, without connecting this with evolu- 

 tion. By using their work we can get a comprehensive 

 view of the development of ammonoids from the most 

 primitive goniatites to the most highly developed am- 

 monites, and thus construct a tentative family tree. 



The simple primitive forms of the Lower Devonian 

 branch out by the end of that age into two distinct 

 stocks, the Prolecanitida and the Goniatitidce, mostly low 

 whorled, involute, with simple sutures and little orna- 

 mentation. Before the end of the Carboniferous some 

 genera have already become ammonitic in the digitation 

 of their sutures, as Popanoceras, Thalassoceras, Pronorites 

 (Plate IV, Figs. 9 and 10), and some have taken on am- 

 monitic ornamentation of the shell, while the sutures 

 remain simple and entire, as Gastrioceras (Plate IV, Fig. 

 n) and Paralegoceras (Plate IV, Figs. 12 and 13). None 

 of these forms, however, are very evolute, and the 

 whorls are mostly rather low. These are all progressive 

 in development, and probably nearly all became radi- 

 cles of more highly specialized groups. No retrogres- 

 sive, or reversionary, goniatites are known. In the 

 Permian Pronorites and its descendants Sicanites and 

 Medlicottia play an important part, the Arcestida are 

 already become important members of the fauna, the 

 Tropitidcz. are just beginning, while the Glyphioceratidce 

 are dying out. Some few genera still persist in the 

 goniatitic stage, but most of them became ammonitic 

 before the Trias was well on. 



In the Trias the important groups are Arccstida, Pina- 

 coceratidce, Tropitida;, Ceratitida, with numerous others 

 less important as members of the Triassic fauna, but of 

 great interest as ancestors of many of the chief families 



