Diener — Critical Phase in History of Ammonites. 121 



This decay is completed in the Noric epoch. From this 

 stage eleven forms of ammonites only have been enumer- 

 ated by Pompeckj, all of them of decidedly Triassic 

 affinities. Five belong to the Noric genus Choristoceras 

 and its subgenus Peripleurites, a phylogerontic descen- 

 dant of the Ceratitidae, whose last whorl became gradually 

 uncoiled. Arcestes, the true leading genus of the Hall- 

 statt limestone, is still represented by two species. To 

 these are added one species of Monophyllites (Mojsvar- 

 ■ites), of Megaphyllites, and a specifically undeterminable 

 representative of Cladiscites, all genera of considerable 

 vertical range. A single newcomer is indicated by Hes- 

 perites, a genus still imperfectly known, which is probably 

 allied to the family of Trachyceratidae. 



It is noteworthy that not a single ancestral represen- 

 tative of Liassic ammonites is recognized in this assem- 

 blage. The discovery of one other genus is to be expected 

 beds of Rhaetic age. This is Phylloceras or, more exactly, 

 with certainty, although it has as yet not been found in 

 Rliaeo phyllites , if this subgeneric designation is extended 

 to all widely umbilicated species of Phylloceras. Rhaco- 

 phylUtes debilis Hau. and R. neojurensis Quenst. are 

 among the most common leading fossils of the upper 

 Noric substage. In the Lower Lias, Phylloceras, Rhaco- 

 phyllites and Euphyllites are remarkable for their rich- 

 ness and variety. The apparent intermittence of Phyl- 

 loceras- in the Rhaetic is tkeref ore purely accidental. It 

 is in reality tke only genus surviving tke general exter- 

 mination of Triassic ammonites. 



Tke importance of tke gradual decline of Triassic 

 ammonites during tke Rhaetic epoch is evident from a 

 comparison with the number of genera in the Carnic and 

 Noric faunae. Those faunae do not contain less than 146 

 genera and subgenera of ammonites, which were reduced 

 to six in the Rkaetic stage. Hyatt was certainly rigkt in 

 speaking of a "culmination of ammonites in tke Upper 

 Trias after a period of uninterrupted progressive evolu- 

 tion from tke early Devonian.'' Botk tke Carnic and 

 Xoric ammonites were highly varied, including forms 

 witk long and short body-chambers, with few and simple 

 clydonitic sutures (Lobiies) and with a very large number 

 of the most complicated sutural elements (Pinacoceras) ; 

 smooth, globose shells with serial lobes (Arcestes) and 



