42 



THE GEOLOGIST. 



neral rule ; whilst others offer remarkable differences, leav- 

 ing at the same time similar characters in the lobes and con- 

 volutions. In the first class may be placed A. Beaudanti, 

 grasianus, quadrisulcatus, semisulcatus, Mayorianus, etc., 

 where I have not observed the slightest difference in hun- 

 dreds of specimens ; but I will only quote a few amongst the 

 species which vary most according to age or sex. If, at the 

 same diameter, the number of ribs or tubercles be gene- 

 rally the same, as in A. Fleuriansianus, denarius, etc., there 

 is at least a marked difference in the greater or lesser degree 

 of compression of the shell. In the second series there are 

 many Ammonites which, when arrived at the same diame- 

 ter, have the ribs more or less salient, or on the contrary at- 

 tenuated, as in A. lautus, Rhotomagensis, varians, Mantellii, 

 cristatus, verrucosus, etc., or a greater number with alter- 

 nating ribs, as we find in A. Mantellii, Rhotomagensis, va- 

 rians, ^stierianus, inflatus, interruptus, Martinianus, mam- 

 milaris, etc. Other modifications which are more rare and 

 entirely exceptional, are those which we principally observe 

 in A. latidorsatus, in which species certain individuals, 

 when arrived at the same diameter, are smooth sometimes, 

 and at others furnished with transverse ribs. This ammo- 

 nite is also one of the few species in which I have observed 

 remarkable differences in the convolutions, in individuals of 

 the same age and diameter. There is therefore no fixed rule 

 as to varieties, as their number is greater or smaller accord- 

 ing to the species. It remains for the observer to search 

 for the true limits of these varieties or species in the inter- 

 nal characters of each. 



Accidental varieties. — This series of modifications gives 

 rise to the most extraordinary changes. They originate from 

 injuries or wounds received by the animal during its exis- 

 tence, which interrupt the regular construction of its shell, 

 entirely remove certain external appendages, and cause new 



