Ic;o CALIFORNIA ACADEMY OF SCIENCES. [Proc. 30 Ser. 



the phylembryonic and ananepionic stages ; but at the 

 second septum, metanepionic, it shows the undivided ven- 

 tral lobe of the Prolecanitidae, a stage which Lytoceras 

 omits. In the paranepionic stage Phylloceras also corres- 

 ponds to some genus like JVannttes, and continues in this 

 stage up to about one and a quarter coils, although even at 

 this early stage it is distinguished from Lytoceras by the 

 tendency to form auxiliary lobes. In the adolescent period 

 Phylloceras also goes through a Mono-phyUiles stage, but is 

 analogous to that group with narrower umbilicus, and 

 numerous auxiliary lobes, probably to the subgenus Mojs- 

 varites. Before the end of the adolescent stage it corres- 

 ponds to Megaphyllites and then by imperceptible transi- 

 tions, narrowing the umbilicus and complicating the lobes 

 until at 10.00 mm. diameter it becomes a full-fledged 

 Phylloceras. 



Thus these two genera come from a common origin, and 

 follow the same paths up to Nannites, where they part com- 

 pany, each going through a stage corresponding to that 

 genus, but to different species under it; both go through 

 Monoj)hyllites stages, but here again analogous to different 

 groups and even different subgenera. There the resem- 

 blance ceases, and they develop into different families 

 probably by the middle of the Trias, for in the upper divis- 

 ion of that formation Megaphyllites and Monophyllites are 

 sharply distinguished from each other. In the life-history 

 of these two genera we have a rare opportunity of observing 

 acceleration of development, and divergence of two nearly 

 related stocks, whose history may be traced from the Paleo- 

 zoic to near the end of the Mesozoic eras. 



Stanford University, 

 California, 



May, 1898. 



