REPORT OF THE DIRECTOR 1919 



67 



in the same direction, namely, toward less branches and more 

 symmetric colonial stocks. 



This development of the Lower Ordovician axonolipons graptolites 

 is thus typically orthogenetic, as the result of persistently acting 

 exterior factors such as the advantages of equal distribution of food 

 to all branches and the attainment of equilibrium of the floating rhab- 

 dosome by symmetric and uniform development of all branches. 



The present writer has distinguished ten lines of development of 

 graptolite species among the New York forms from Clonograptus 

 and Bryograptus through Tetragraptus to Didymograptus, which can 

 be traced to at least four different species of Bryograptus and Clono- 

 graptus (Goniograptus). There should then be recognizable here 

 and in time named at least four generic groups of Tetragraptus and 

 ten of Didymograptus. 



Taking our example of the Lower Ordovician graptolites of New 

 York, we get the following phylogenetic diagram of these earlier 

 graptolites. 



It would lead to a bewildering and complex nomenclature if v/e 

 should attempt to recognize all these racial lines and stages by sep- 



DidvMOgraptus 

 shge or 

 Circulus 



(V 



Tetragraptus 

 sta^ or 

 Circulus 



;7 ::8 ;^9 i 1)0 i\, 



Miiltiramous 

 stages 



CloTiograptus 

 Bruograptus 

 DichograpTus 



XSpcsciea 



Clcmograptu 



Fig. 4 Phylogenetic diagram of the American axonolipous graptolites; 

 the figures representing the number of species. 



