AMMONiTHS — ^OOLOGICAL^ ETC. 



69 



^les. The ammonites, which had become changed into an 

 oxide or hydrate of iron^ have principally furnished me with 

 the means^ by breaking the shell and taking out the whorls 

 successively, one after another, up to the first stage of growth 

 of each species. It is thus that after many researches and 

 comparisons, and after sacrificing a great number of speci- 

 mens, I have determined that the growth of ammonites, may 

 generally be apportioned into five principal modifications 

 according to age, as follow : 



First modification which I shall name the 'Embryo period 

 {periods emhryonnaire). As a general rule, ammonites com- 

 mence, at a very young period (when only a few millimetres 

 in diameter), by being entirely smooth, and by having the 

 back round, even when they ought, at a more advanced 

 period, to have a keel or sharp back, as we find in A. bifrons^ 

 (Walcotii) cordatus, cristahis, &c. and then they might all 

 be confounded together, if the convolutions were not difi'e- 

 rent in this embryo state. We can conceive nevertheless, 

 that this character, which is very limited, being aided by no 

 external appendage, nor even by a keel, the number of dis- 

 tinct forms at this age is very small, and that their distinction 

 into species is almost impossible.* 



Second modification which I shall name the first period of 

 growth (premiere periode d'accroissement). To this smooth 

 state, of the first age of ammonites, succeeds another amongst 

 those species which become furnished with external orna- 

 ments; for as regards ammonites which remain always 

 smooth, they necessarily continue their growth in a uniform 

 manner. In this period of growth, the appendages which 

 we observe on the ammonites, that ultimately become 

 striated or ribbed, are the presence of salient portions or 



* M. Milne Edwards has observed this singular character, amongst the 

 Crustacea. So that it may prove a general law in zooloj^y. 



