170 PAL/EONTOLOGY 



inspired and started on his work by Louis Agassiz, con- 

 fined himself to the ammonites of one small division o 

 time and divided them with much greater minuteness. 

 Thus it happened that one of Waagen's genera (Arietites) 

 was equivalent to a whole family of Hyatt's. The 

 American worker also initiated the method of using 

 individual development (ontogeny) as an essential criterion 

 in classification. This double start led to some confusion 

 in the naming of Jurassic ammonites, which Mojsisovics 

 and Gemmellaro, working on the almost untouched mass 

 of Triassic and Permian species, were able to avoid. 

 Numerous later workers have added greatly to the recog- 

 nized number of Ammonite genera work which is not 

 always greeted by stratigraphers with the thanks which 

 it deserves, but which is inevitable once the evolutionary 

 conception of a genus is accepted. 



GENERAL HISTORY OF THE AMMONOIDEA. 



As so often happens, the apparently most primitive 

 genera are not the earliest. The orthocone Bactrites 

 indeed is doubtfully reported from the Silurian, but is 

 not known in the Devonian until nearly the end of the 

 Middle epoch of that period ; while Gyroceras [Mimoceras], 

 which starts as a gyrocone and continues as an ophiocone, 

 is known from the Middle Devonian only. At the very 

 beginning of the Devonian there appear two genera in the 

 ophiocone stage, Anarcestes (Fig. 47, a) and Agoniatites 

 (Fig. 41, b), the very simple sutures of which have led 

 most palaeontologists to place them in a family Nautilinida, 

 though they might perhaps better be regarded as at the 

 base of separate other families. Towards the end of 

 Middle Devonian time, a new family Magnosellaridce 



