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 Prolecanitide and the Gontatitide, 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.. 
11) 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 Arcestide are 
already become important members of the fauna, the 
Tropitide are just beginning, while the Glyphioceratide 
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 Arcestide, Pina- 
coceratide, Tropitide, Ceratitide, with numerous others 
less important as members of the Triassic fauna, but of 
great interest as ancestors of many of the chief families 
