PALEONTOLOGY. 403 



hook-shaped Hamites, and the various groups were recognised 

 in the classification by the use of descriptive adjectives. Buch 

 also gave a clear exposition of the progressive complication 

 in the sutural lines which could be observed in following 

 the phylogeny of the Ammonitidae from the Palaeozoic epochs 

 through the Mesozoic, and showed how a surmise might be 

 made respecting the age of an Ammonitid genus from the 

 relative degree of complexity in the sutural limits. 



Buch's three sections, Goniatites, Ceratites, and Ammonites, 

 were defined by subsequent writers more in harmony with 

 zoological definitions of the group, but the discovery of the 

 rich Triassic fauna of St. Cassian showed that the distinctions 

 between these sections were by no means so sharp as had 

 been supposed. Buch's work undoubtedly gave a new impulse 

 to the study of fossil Cephalopods. The middle decades of 

 the nineteenth century saw the publication of a large number 

 of memoirs, elucidating the genetic relationships of the Palaeo- 

 zoic and Mesozoic genera. The erection of new genera and 

 species went on rapidly, and the necessity began to make itself 

 felt for a further sub-division of the typical genus Ammonites. 

 Barrande, Hall, and other authors had already divided the 

 original Nautilites into a number of genera. 



The decisive step of sub-dividing the Ammonites was 

 ventured by Suess in 1865. In a short memoir on the 

 organisation of the Ammonites, Suess converted the adjectival 

 nomenclature of the individual groups of species into names 

 of genera (Phylloceras, Lytoceras, Arcestes), and pointed out 

 that in addition to the sutural line, external form and orna- 

 mentation of the shell, there were other features of systematic 

 value, such as the margin of the mouth and the length of the 

 chambers. A similar reform was advocated by Alpheus Hyatt 

 in his memoir on the Liassic Ammonites (1869). The pre- 

 vious nomenclature of families was discarded by Hyatt, and 

 numerous new genera were erected, whose limits were much 

 more narrowly defined than had been customary. As one 

 might have expected, the new tendency met at first with strong 

 opposition, but it was supported and followed by Laube, 

 Zittel, Mojsisovics, Waagen, and Neumayr 



Waagen in 1871 combined the Ammonitid genera in eight 

 groups, attributing great importance to the presence or absence 

 of the shell plates termed "Aptychus" and " Anaptychus," 

 and to the particular structure of these remains. Neumayr in 



