13G 



THE GKOLOGIST. 



the whorls simply in contact, cyhndrical, smooth, or marked 

 at intervals by points of interruption; and always having 

 two lobes on each side {Am. subfimbriatus, Honnoratianus, 

 quadrisulcatus, striatisulcatus, strangularis, JuilletL All 

 these Ammonites have, otherwise, the same external fades. 



2. — Still, amongst the Ammonites having the lobes formed 

 of equal parts, there exists a series where the whorls, more 

 or less enveloped, are furnished, on the back, with several 

 ranges of tubercules, one of which is central ; the number of 

 equal lobes and saddles is always above two {A. Rhotoma- 

 gensis, Fleuriansianus, JVooIgari, Verneuilianus, ^c). All 

 have likewise the same external appearance. 



3. — Amongst the Ammonites having the lobes formed of 

 unequal parts, but whose saddles are equal, there is a series 

 which has the dorsal lobe longest and very wide, belonging to 

 the species having the whorls compressed, provided with a 

 salient dorsal keel, and having the mouth entirely pointed 



Roissyanus, BouchardianuSf inflatus, Delarnei, ^c). 



4. — There exists still another series amongst the Am- 

 monites, having both the lobes and saddles formed of unequal 

 parts, whose dorsal lobe is very narrow and exceedingly 

 short, belonging to the species having their whorls com- 

 pressed or angular, furnished, on the dorsal line, with a very 

 deep furrow, in which the dorsal lobe is placed {A. tubercu- 

 latus, lautus, auritus) . 



These four examples, taken from species having the lobes 

 and form entirely different, are, I think, sufficient to prove 

 that there is evidently an affinity between the external and 

 internal forms, and that, therefore, the form of the lobes 

 acquires so much the greater importance in the classification 

 of the species into groups, as it is found to coincide with the 

 external ornaments with which Ammonites are covered, and 

 especially with the form of the ])ack. The intimate union 

 of these two series of characters, will enable us to form very 

 distinct groups in the grand genus of the Ammonites. 



