THE EXTINCTION OF SPECIES AND OF GROUPS 241 



straight shells or Orthoceraconi belonging to differ- 

 ent phyla from the Cambrian down to the Trias. 



Concerning the shells of the Ammonoids, the 

 evolution of the coil passes also through the stages 

 of youth, maturity, and senility, which we meet with 

 alike in nearly all the innumerable branches of this 

 group. Hyatt has given the name of Bactriticoni 

 to the straight shells such as the Bactrites, that of 

 Mimoceraconi to the loosely coiled shells of the 

 type Mimoceras, that of Ammoniticoni to the nor- 

 mal adult stage characterized by a closely spiral 

 coiling ; finally, the name of Torticoni to all senile 

 cases of asymmetric coiling. 



It has been known for a long time that the 

 evolution of the line of suture allows us in the 

 same way to establish among the Ammonoids, stages 

 with increasing complication which are found, at 

 different epochs, in all the branches. 



Among the Vertebrates analogous observations 

 have been made in different groups ; for instance, 

 in the Ganoid Fishes. The primary types of this 

 order present youthful characteristics which mani- 

 fest themselves by an ossification, either nil or 

 very little advanced, of the vertebral column, 

 which remains soft and in the state of embryonic 

 tissue. A little later, at the Liassic epoch, this 

 ossification invades, little by little, the vertebrae, 

 and towards the middle Jurassic the family of the 

 Leptolepidce has acquired an ossified vertebral 

 column similar to that of our existing bony Fishes. 

 The Amphibians present, on their side, at the 

 Carboniferous and Permian periods, stages of ossi- 

 fication quite comparable to those of the Ganoids. 



