INDIVIDUAL AND PAL2EONTOLOGICAL EVOLUTION 261 



of the same Psiloceras planorbis with a more com- 

 plicated suture. The embryonic whorls of all forms 

 of this branch will, therefore, be smooth ; then ribs 

 and a strong keel appear in the Arnioceras ; this 

 keel finally becomes accentuated, and in the enor- 

 mous Coroniceras of the Sinemurian, is bounded by 

 two furrows. A subdivision of this branch, in 

 which the whorls become gradually less spiral and 

 the shell flattens, brings us, through the intermediary 

 of the Agassiceras, to the discoid and sharp-edged 

 shells of the Oxynoticeras . Again, it is by studying 

 the individual evolution of the suture line that G. 

 Sayn has proved the unforeseen ancestral connec- 

 tions of these smooth and sharp-edged shells of 

 the Oxynoticerata of the Jurassic with the very ele- 

 gantly ornamented shells which constitute the in- 

 teresting little family of the Barremian Pulchel- 

 liidce. It may be hoped that at no very distant 

 future specialists may arrive, by the aid of this 

 method of individual ontogeny, at tracing with 

 exactness the innumerable phyletic branches of 

 the great group of the Ammonoids. 



For the study of the evolution of the Lamelli- 

 branchs, Felix Bernard has employed a method 

 somewhat different from the one I have just indi- 

 cated as regards the Ammonites. He has examined 

 with attention the embryonic shells (nascent)* found 

 in extraordinary abundance in certain Tertiary de- 

 posits, such as the Miocene sands of Saint Paul de 

 Dax, and has been able to follow the modifications 

 of the hinge, and the position of the ligament and 

 of the adductor muscles common to certain genera 



* Naissain. Name applied to youiig oysters still in their beds. ED. 



