260 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



advantage of the characteristics of the suture line 

 in the embryonic and ephebic whorls of the Ammon- 

 ites, to discover the ancestral relations of genera 

 apparently far apart from each other, as shown by 

 the characteristics of the adult whorls. Among the 

 most remarkable attempts in this line we must 

 quote the researches of Karpinsky on the phylogeny 

 of the Prolecaniditce family, a work in which the 

 Russian scholar has followed with the greatest care 

 and through several branches the ontogenic de- 

 velopment and the order of apparition of the genera, 

 from the Ibergicerata and the Prolecanitce of the 

 Devonian down to the Lecanitce and Noritce of the 

 Triassic epoch. In the same way Hyatt, utilizing 

 at once the development of the compartments, 

 the coiling of the shell, and the details of its ex- 

 terior ornamentation, has essayed to trace the 

 evolution of the genera of the great family of the 

 Arietitce. Joined, perhaps, to the Triassic Gymnitce, 

 the radical form of the group is the genus Psilocems, 

 of the Hettangian stage, from which would be 

 derived two branches of Arietites. The first or 

 plaited branch has for ancestral form a plaited 

 variety of Psiloceras planorbis, which passes by 

 way of the contraction of the plaits and their 

 transformation into prominent ribs, to the succes- 

 sive species of the genus Schloiheimia. This same 

 branch gives by bifurcation another series charac- 

 terized by the apparition on the median line of 

 a large keel between two longitudinal furrows. 

 This series gives successively the Caloceras, and 

 then the Vermiceras of the lower Lias. The second, 

 or smooth branch, is derived from a smooth variety 



