122 Diener — Critical Phase in History of Ammonites. 



extremely flattened shapes (PompecJcjites) ; shells 

 exhausting* almost every possible combination of sculp- 

 ture from the most graceful ornamentation (Acanthin- 

 ites) to stout ribs (Heraclites) and profusely tuberculated 

 costations (Trachyceras). 



The close of the Rhaetic epoch is marked by the final 

 disappearance of all Triassic types, excepting Phyl- 

 loceras. Primitive and highly specialized forms were 

 equally subjected to this general extermination. 



In the eastern Alps the beds of the lowest Lias follow 

 above those containing a Rhaetic fauna without any uncon- 

 formity. There is no trace of a hiatus nor of any 

 diastrophic movement between the two groups. Never- 

 theless the ammonite fauna of the lowest zone of the 

 Mediterranean Lias is entirely different from that of the 

 Upper Trias. The first impression of this Liassic fauna 

 is the sudden introduction of a large number of types 

 which are only a little less manifold and diversified than 

 those of the Upper Noric, but do not exhibit any phylo- 

 genetic affinities with them. We are indebted to F. 

 Waehner for their careful and detailed examination. 



There is little doubt that the extinction of the different 

 phyla of Triassic ammonites prepared the way for the 

 evolution of a new and vigorous stock, which originated 

 from the genus Phylloceras, the only one which connects 

 the faunae of the Triassic and Liassic periods. Phyl- 

 loceras is the ancestor of the two leading families of the 

 lowest Lias, the Arietitidae and Lytoceratidae. Waehner 

 and Pompeckj have demonstrated their intimate relation- 

 ship with Psiloceras, the most primitive element of the 

 Arietitidae. Together with Psiloceras, more specialized 

 types of the Arietitidae : Mgoceras, Schlotheimia, Ariet- 

 ites, make their appearance in the deepest zone of the 

 Lias. But they are comparatively rare, Psiloceras 

 remaining the predominant genus in this and the follow- 

 ing life-phase. All these genera are linked together most 

 closely with the ancestral Psiloceras. 



Of equal moment is the sudden appearance of the Lyto- 

 ceratidae in the Lower Lias, where they are represented by 

 the genera Lytoceras, Ectocentrites and Pleuracanthites. 

 Forms transitional between Pleuracanthites and Psilo- 

 ceras have been described by Waehner. Thus Phyl- 



