JAMES PERRIN SMITH 21 



The best genetic series of Ammonoids are found in the Paleozoic 

 and early lilesozoic. There we get a nearly unbroken series of adult 

 forms that show by their sequence and intergradation that they are 

 genetically connected. In most of these genera we have also their 

 individual development repeating the ancestral history, not the whole 

 history distinctly, but that part nearest to them most positively. Such 

 a series leads from the Glyphioceratidae through Gastrioceras of the 

 Carboniferous, to Columhites of the Lower Triassic, and up to the 

 Tropitidae of the Upper Triassic. The writer is strongly of the opinion 

 that this phylum will yet be traced still higher, into the Arietidae and 

 Stephanoceratidae of the Jurassic. 



Such a series is seen also in the Ceratitoidea. The parent, or 

 radicle, of this group, Lecanites, as we know it in the Triassic, is still 

 virtually a Goniatite, with simple unbranched septa, and repeats the 

 race history of the Devonian Gephyroceratidae. The more primitive 

 members of the Meekoceratidae of the Permian and Lower Triassic repeat 

 this part of the history, and all show a distinct Lecanites stage. The 

 earlier members of the Ceratites are still nearly smooth, and intergrade 

 with the later members of the Meekoceratidae, still showing in their 

 youth a decided reminiscence of Lecanites. From the earlier and simpler 

 smooth Ceratites there branched out two groups of rough shelled forms, 

 one leading towards the keeled Ceratites, group of C. trinodosus, the 

 other leading through the group of C. hosnensis to the Trachyceratea, 

 all connected by series of mature forms, but not showing their phylogeny 

 in their ontogeny, except in cases of arrest of development and 

 retardation. 



The division between Permian and Triassic was a deadline for most 

 Paleozoic groups; on the one side we have rugose corals and tabulates, 

 on the other the modern Hexaeoralla; on the one side Productus and 

 Orthis, on the other a predominance of Terebratulacea and Rhynchonel- 

 lacea; on the one side Palseocrinoidea, on the other Neocrinoidea. It 

 is not so with the Ammonoids, for in them there is a nearly perfect 

 transition, not with any species, but with a number of genera surviving 

 from Permian into the Lower Triassic, and with many getting across 

 the line so little modified that, while we call them by different generic 

 titles, they are still virtually the same as their Paleozoic forebears. 



The following genera survive from Permian into the Triassic: 

 Otoceras, Hungarites, Xenodiscus, Xenaspis, Pronorites, Medlicottia 



