82 



THE GEOLOGIST. 



with the characters which serve us as a base for the distinc- 

 tion of groups, they being, as we shall see, always relative to 

 the number of lobes of the Septa. 



In speaking of the differences of the sexes I have also 

 remarked that the envelopment of the whorls, and for the 

 same reason, the size of the last one, with respect to the 

 diameter, varies according to sex and that the whorls, which 

 are larger in the males than in the females, are almost al- 

 ways so on account of the compression of the shell. 



Size of Ammonites. — The growth of Ammonites stops at 

 different periods, and has no connection with the complica- 

 tion of the external ornaments, or the internal characters of 

 the septa. I am not even certain that it would be easy to 

 establish any relation between the volume which some at- 

 tain and the probable duration of their existence ; nothing 

 can reveal to us the time which elapses between the forma- 

 tion of each Septum or external point of termination, even 

 were it identical in all the species. It is therefore a ques- 

 tion which will doubtless remain always undecided, and in 

 every instance quite hypothetical. It is not so as to the 

 size proper to each species, and from that compared with 

 external and internal modifications, always being careful to 

 render this comparison subordinate to the varieties arising 

 from age, which I have already noticed, I can affirm that 

 each species appears to attain a certain diameter which it 

 never exceeds. For instance in the Cretaceous formation : 

 1st. A. striatisulcatus, Belus, Calypso, picturatus, pretiosus, 

 verrucosus, crassicostatus, asperrimus, Bravasianus and 

 Itierianus, ^c, always retain a small size, that is to say a 

 diameter below three centimetres. 2nd. A, Bufrenoyi, li- 

 gatus, Parandieri, macilenfus, Emerici, Duvalianus, tuber- 

 culatus, regularis, ^c, are larger, but do not exceed five 

 centimetres ; 3rd. A. Asterianus, fascicularis, subfimbriatus, 

 latidorsatus, Denarius, falcatus, ^c, and many others 



