a neio Classification of Ammonites. 371 



little in front, on the convexity of the precedinp^ turn. It has 

 been observed in the Goniatites by Keyserling^ in many 

 Silurian Cephalo])oda by M. Barrande, and in the Clymenice by 

 M. Giimbel ; M.Quenstedt and M.von Hauer have recognized 

 it in the Ammonites of the group Arcestes] M. Laube in 

 A. {Phylloceras) Jarha^\ M. Suess in Clydonites delphi- 

 nocephalus. 



This layer extends to a larger or smaller part of the interior 

 of the shell ; it becomes gradually effaced and disappears with 

 growth. It appears to be wanting in the genera which present 

 contractions. The Ammonites in which it is found (Arcestes 

 and Phylloceras) are also those which have varices ; by these 

 characters they approach certain palaeozoic Cephalopoda ; they 

 are also those which appeared first*. 



The great differences in the structure of the Aptychi indicate 

 considerable differences in the structure of the nidamentary 

 glands, and consequently in the entire organization of the 

 Ammonites. The various characters which we have enume- 

 rated seem fully to justify a division into genera of the fossils 

 of this great group. This new classification is based, then, 

 first of all on the structure of the nidamentary gland ; next, 

 on the length of the chamber of habitation ; in the third place, 

 on the form of the latter and of the aperture, the septa, and the 

 ornaments. The general form of the shell, to which the older 

 classification attached great importance, seems to be a very 

 variable character, and, in consequence, a secondary one. It 

 is upon these principles that M. Waagen has based the 

 following table : — 



A. NlDAlVIENTARY GlAND WITHOUT SOLID INTEGUMENT. 



Chamber short ; appendage ventral Phylloceras, Suess. 



Chamber short ; appendage dorsal Lytoceras, Suess. 



Cliamber very long (14 to 2 turns) Arcestes, Suess. 



? Chamber short ; apertural margin falciform, 



with the appendage ventral ; ornaments of 



the same kind as those of the Argonaut . . , , Trachyceras, Laube. 



B. Nidamentaby Gland with a solid integument (Aptychus). 

 I. Gland simple, not divided, ivith : — 

 1. Integument homy (Atutptycktis). 

 Chamber very long (1 to 1^ turn) ; apertural 



margin with a pointed ventral appendage . . Arietites, Waag. 

 Chamber from § to 1 turn; apertural margin 



with a rounded ventral appendage ^goceras, Waag. 



Chamber short (^ to § tmii) ; apertural margin 

 with a long ventral appendage Amaltheus, Montf. 



* These Ammonites are abundant in the Trias. M. Waagen has lately 

 discovered some in the Carboniferous formation in India. 



24* 



