82 



CEPHALOPODA. 



which are now generally accepted ; and it is the less neces- 

 sary to do so, as it mnst be admitted that it is generally 

 impossible to refer particular specimens to these sections, 

 unless they are in a state of unusually complete preserva- 

 tion, or unless the observer be provided with a very exten- 

 sive suite of examples of a given form. It may be of value, 

 liowever, to reproduce the classificatory table of the sections 

 of Ammonites adopted by M. E. Favre (see Bull, de let Soc. 

 Giol. dc France, ser. 3, vol. i. p. 353) : — 



SUBDIVISIONS OF AMMONITES. 



Section I. — No Apttchus. 



«. Body-chamber short ; the upper side of \ 

 the aperture prolonged. S 



b. Body-chamber short ; the aperture pro- ) 



longed below. ) 



c. Chamber very long, extending over one ) 



and a half to two whorls. ) 



d. Body - chamber short ; margin of ap- ^ 



erture falciform ; mouth prolonged ( 

 above ; ornaments of the shell like i 

 those of the Ar"onau.t. J 



Phylloceras. 

 Trias to Cretaceous. 



Lytoceras. 

 Trias to Cretaceous. 



Arcestes. Trias. 

 Pinacoceras. Trias. 



Trachyceras. 

 Trias. 



Section II. — An Aptychtjs present. 



A. The Aptyclms corneous and undivided (Anaptychus) 



a. Body-chamber extending over one whorl •\ 



and a half or two whorls. The upper f 

 margin of the aperture prolonged into ( 

 a point. ^ 



b. Body-chamber extending over two-thirds \ 



of a whorl or one whorl ; the upper ^ 

 margin of the aperture prolonged into I 

 a roiinded extension. ^ 



c. Body-chamber short, extending over one- ^ 



half or two-thirds of a whorl ; the up- f 

 per margin of the aperture prolonged C 

 greatly. 

 ((/. The Aptychis in one piece but calcareous ; shell unknown. 

 elms Numida. Cretaceous.) 



B. Aptyclms calcareous, of two pieces : — 

 a. The Apitychus furrowed externallj-. 



Arietites (Waagen). 

 Triassic and Jurassic. 



JEgoceras (Waagen). 

 Triassic and Jurassic. 



Amaltheus. 

 Triassic to Cretaceous. 



Apty- 



