AMMONiTES — ZOOLOGICAL, ETC. 



71 



par. denarius, splendens, bicurvatus, inflatus, Camatteanus, 

 Vernenilianus, and a great number of species from the Juras- 

 sic formation, such as A. Lamberti, Banksii, &c.) But the 

 ammonites do not preserve these external characters bej^ond 

 a certain period, the external appendages, although remain- 

 ing virtually the same^ become diminished in size, the salient 

 portions are less strongly marked and are gradually effaced ; 

 the shell, although still growing is no longer so perfect ; it 

 degenerates. 



Fourth modification. First period of degeneration {pre- 

 miere periode de degenerescence) . — Most Ammonites re- 

 main^ until the period of their greatest growth, in the third 

 state of modification {A, interruptus, Denarius, Astierianus, 

 auritus, mammillaris, ^c.) the ribs only become more 

 apart, and the tubercules more prominent. It frequently 

 happens also, that at a more advanced age, the ribs and striae 

 of the back gradually become effaced^ and finally disappear, 

 whilst the lateral tubercules, where the ribs of this portion 

 are wide apart, become much more salient, and remain thus 

 isolated, during a longer or shorter period of growth. This 

 period which corresponds in all to the first stage of growth, 

 since it represents the Ammonites in the state in which they 

 were at that age, may be called the first period of degenera- 

 tion, the Ammonites already losing some of their external 

 characters. {A. dispar, Clementinus, radiatus, bicurvatus, 

 Lewessiensis, of the Cretaceous period, and in the Jurassic 

 Formation, A. Lamberti, muiabilis, Plicomphalus, triplica- 

 tus, decipiensy Murchisonce, Banksii, insignis.) 



Fifth Modifications, which I shall call the last period of 

 degeneration ( dernier e periode de degenerescence.J —-Amongst 

 the Ammonites which I have named, there are many which 

 attain their greatest known diameter, in the last state; but 

 there are also a great number, which, after having retained 

 this modification for a longer or shorter period, ultimately 



