74 



THE GKOLOGIST. 



and the study of the lobes and saddles, for the purpose of 

 fixing the limits of varieties such as I have described them, 

 has, as we shall see hereafter, furnished me with a great 

 number of additional observations. 



MOUTH OF AMMONITES. 



Among the external characters of Ammonites, there is one 

 which although it has already been the subject of notice, still 

 requires much examination : it is that of the Aperture or 

 mouth. M. Defrance first figured some perfect margins^, 

 and I then had the pleasure of furnishing him with some 

 materials ; but, from a want of the means for study, the iso- 

 lated facts collected in this first essay, cannot be applied to 

 general views, and these remains being rare in collections, 

 no one has since undertaken the enquiry. After having 

 examined a great number, I have I think arrived at interest- 

 ing results as to the connexion which exists between the 

 form of the aperture and that of the dorsal keel, and the 

 natural sections into which the Ammonites may be classed. 

 I find that the modifications of the margin are of two kinds. 

 They follow the external lines of growth and the curve of 

 the ribs at all ages, and we can then see them externally on 

 the shell; or otherwise they have no connexion with the or- 

 dinary lines of growth and curve of the ribs, thus constitut- 

 ing a part entirely unconnected with the forms of the other 

 portion of the shell. In the first instance I shall name them 

 regular apertures {bouches constantes) ; in the second irregu- 

 lar apertures {bouches moment anees). 



Regular apertures or mouths may be classed in three 

 series ; 



In the first class we may place such as are provided with 

 * Dictionnaire des sciences naturelles. 



