INDIVIDUAL AND PALJEONTOLOGICAL EVOLUTION 257 



of young forms whose preservation during the act 

 of fossilization is more uncertain. 



Certain groups, however, and particularly the 

 Gastropod and Cephalopod Molluscs, retain in- 

 teresting traces of their youthful stages, at least 

 as far as regards the characteristics of the shell, 

 thanks to the construction by the animal, in the 

 course of its individual growth, of spiral whorls, or 

 successive dwelling-chambers, the modifications of 

 which it is fairly easy to study. It is open to us, 

 for this purpose, either to examine the individuals 

 of various sizes, and, consequently, of various ages, 

 in one species, or which is a still more effective 

 process to break open the shell, in order to study 

 its internal windings, to take it to pieces, so to 

 speak, room by room, from the embryonic whorls 

 down to the adult and even the senile ones. On these 

 lines the shells of the Ammonites, in the hands of 

 Sandberger, Keyserling, Hyatt, Branco, Karpinsky, 

 Mojsisovics, etc., have given exceedingly interest- 

 ing results, both from the point of view of the 

 general evolution of the group, and from the genetic 

 relations of genera and families. 



Attention has been specially directed to the 

 development and progressive complication of the 

 partitions, that is to say, the suture line which marks 

 the separation of the different chambers. In the 

 most primitive Ammonoids the first partition 

 formed immediately after the initial ovoid chamber 

 is straight or hardly marked with a slight sinus, 

 and thus reminds one of the adult partitions of the 

 Nautilidae ; it is the saddleless type of Branco. 

 This very simple type of partition only persists, 



