n 



THE GEOLOGIST. 



change it. The ribs or lateral tubercuks separate more and 

 more^ and gradually becoming lower^ ultimately disappear 

 entirely, leaving the shell as smooth, in its last Whorl as in 

 the first, i. e. the embryo state. It is then, at the maximum 

 of its growth, returned to what it was at its birth. {A. Dis- 

 par, Clementinus^ bicurvatus, Lewessiensis, radiatus, in the 

 cretaceous rocks, and Lamberti, mutabUis, Murchisonce, 

 insignis, Banksii, <Sfc., of the Jurassic Formation.) 



In conclusion, according to age, amongst the Ammonites, 

 there is evidently a limited number of modifications of 

 groivth and degeneration. These modifications are not the 

 result of chance, but are the consequences of certain and re- 

 gular periodical metamorphoses, which the greater part of 

 Ammonites undergo and which operate ipxvariably in a regu- 

 lar order of succession. Each, in fact, smooth at an early 

 age, becomes covered at a more advanced period, in the 

 course of its growth, by tubercules around the Umbihcus, 

 then with ribs, striae or tubercules on the back. It is 

 then in the adult state. Arrived at the maximum of exter- 

 nal perfection, all these ornaments commence to change ; it 

 degenerates. The strise and dorsal ribs disappear in the 

 first place ; it then loses the lateral ribs and tubercules and 

 becomes, in its old age {vieillesse), as simple in external ap- 

 pearance as it was in the embryo period. 



The enormous differences between the internal cast and 

 the external appendages of the shell, the natural and acci- 

 dental varieties, the varieties arising from sex and especially 

 the great number of variations, arising from age, not being 

 sufficiently known, we may easily conceive into what inextri- 

 cable confusion the great Genus of the Ammonites has 

 been thrown, each person wishing to establish species on iso- 

 lated specimens, without having previously studied them in 

 detail, and without knowing on that account, when to stop. 

 We can also easily conceive, that even if we had recognized 



