Diener— Critical Phase in History of Ammonites. 123 



loceras was destined to give rise to all Lower Triassic 

 ammonites by the intervention of Psiloceras. 1 



Tims an aspect quite different from that of the Upper 

 Trias is given to the ammonite fauna of the Lower Lias. 

 Not one of the numerous and diversified genera of world- 

 wide distribution, belonging to the families of Arcestidse, 

 Cladiscitidse, Pinacoceratida?, Haloritidae, Tropitidae, 

 Didymitidse, Ceratitidae, Tirolitidse, and Trachyceratidse is 

 represented in the latter. Their place has been taken by 

 Arietitida? and Lytoceratidae. Phylloceras, which never 

 played an important part in the fauna of the Upper Trias, 

 was the only survival and was destined to become the 

 ancestor of all Liassic ammonites. 



In direct opposition to these facts, Steinmann denied 

 the extermination of Triassic ammonites at the close of 

 the Ehaetic epoch. His reconstruction of a phyletic tree, 

 in which Macrocephalites is branching off from Juvavites, 

 Sphceroceras from Halorites, Harpoceras from Disco- 

 tropites, Desmoceras from Arcestes, P a cliy discus from 

 Cladiscites, need not be discussed here. It means toying 

 with possibilities, the reality of which can never . be 

 proved. 2 One of his critical arguments, however, 

 deserves consideration. He believes the paheontological 

 record not to be sufficiently perfect to prove a real decline 

 of the Triassic ammonites during the Silastic epoch. It 

 is true that cephalopod-bearing strata of Ehaatic age have 

 scarcely been discovered up to now outside the north- 

 eastern Alps. But here they are as rich in ammonites as 



1 In connecting Psiloceras with Phylloceras (Bhacophyllites) I am fol- 

 lowing J. F. Pompeckj ? s view, which has been set forth by this author in his 

 memoirs, "Xote sur les Oxynoticeras clu Sinemurien du Portugal, etc." 

 (Comm. serv. geol. Portugal, VI, 1906-1907, p. 332) and "Zur Kassenper- 

 sistenz der Ammoniten' ; (3. Jahresber. d. niedersachs. Geol. Ver. Hanno- 

 ver, 1910, p. 82). E. v. Mojsisovics prefers to consider a specialised type 

 of MonophylJites (Mojsvarites phuwrboides Winkler) as the ancestor of 

 Psiloceras. Winkler's description and illustration, on which this suggestion 

 has been based, are not absolutely reliable, and the type-specimen itself has, 

 unfortunately, been lost. 



It makes, however, little difference, whether the one or the other view is 

 adopted, Mojsvarites itself being closely related to the Phylloceratida?. 

 According to Pompeckj, one genus only, Phylloceras, persists throughout the 

 Triassic and Jurassic periods. In following E. v. Mojsisovics we have to 

 record, simultaneously with the decline of Monophyllites, the first appearance 

 of a new and transitional form, connecting this genus with Psiloceras, the 

 undoubted ancestor of all Arietitida. 



-Its onlv advocate is O. Wilckens (Xaturwiss. Wochenschrift, X. F., X., 

 Xo. 45, Jena, 1911, p. 20). 



