516 SL QWOIIES SHOU, iS) H(QYOVIIN TGS 
was known long ago; these naturalists knew, too, that ammon- 
ites went through a goniatite stage of growth, without connect- 
ing this with evolution. By using their work we can get a com- 
prehensive view of the development of ammonoids from the 
most primitive goniatites to the most highly developed ammon- 
ites, 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 Prole- 
canitide and the Goniatitide, mostly low whorled, involute, with 
simple sutures and little ornamentation. Before the end of the 
Carboniferous some genera have already become ammonitic in 
the digitation of their sutures, as Popanoceras, Thalassoceras, 
Pronorites, and some have taken on ammonitic ornamentation of 
the shell, while the sutures remain simple and entire, as Gastrio- 
ceras. None of these forms, however, are very evolute, and 
the whorls are mostly rather low. In the Permian Pronorites 
and its descendants Stcanites and Medlcottia play an important 
part, Arcestide are already become important members of the 
fauna, the 7ropitide are just beginning, while the Glyphioceratide 
are dying out. Some few genera still persist in the gontiatitic 
stage, but most of them became ammonitic before the Trias was 
well on. 
In the Trias the important groups are Arcestide, Pinacocera- 
tide, Tropitide, Ceratitide, with numerous others less impor- 
tant as members of the Triassic fauna, but of great interest as 
ancestors of many of the chief families of the Jura and Creta- 
ceous. In the Jura these ammonites reached their acmre, 
branching out into very many families and subfamilies, increas- 
ing usually in complexity of sutures and variety of ornamenta- 
tion. Inthe Cretaceous they gradually declined, dropping off 
one at a time until all were gone. The total number of Ammon- 
oidea now described reaches about 5000, of which only a few 
hundred belong to the Palzozoic goniatites, the others belong- 
ing to the ammonites of the Carboniferous, Permian, and Meso- 
zoic. Later than this no ammonoids are known. 
Only simple radicles or stocks persist, but from time to time 
