GRAPTOLITES OF NEW YORK, PART 1 553 



homologous organs, though the mode of the formation of the virgula within 

 the walls of the sicula of Diplograptus suggests that this virgula also was 

 originally but a thickening of the periderm — of the nemacaulus however — 

 and only afterward became more or less separated as a rod. 



Since it is obvious that our stock of observed facts is not yet sufficient to 

 solve either the problem of the relationship of the two largest divisions of 

 graptolites, the orders Axonolipa and Axonophora, or that of the phylogenetic 

 connection between the two large groups of graptolites ^vhich Freeh has 

 united under the caption Axonolipa, viz the Dendrograptidae and Dicho- 

 graptidae, it appears that the present status of our knowledge of the 

 differences between the Dendrograptidae, Dichograptidae and Axonophora 

 would be best expressed by still recognizing all of them as independent 

 orders. 



c Phylogeny of the genera and species. Speculation as to the possible 

 phylogenetic relatione of the genera of the Dendroidea, Dictyonema, Desmo- 

 graptus, Dendrograptus, Callograptus and Ptilograptus would be altogether 

 premature, since these groups are based thus far largely on external char- 

 acters which, in view of the complicated internal structure observed in 

 several species, are probably inadequate to a correct understanding of their 

 natural relations. 



The problem of the phylogeny of the Dichograptidae has been the 

 subject of investigation by Nicholson and Marr, whose work has been ably 

 continued by Elles. 



Nicholson and Marr [1895, p.529] observed that in the more ancient 

 types of graptolites the thecae are comparatively simple, while in the later 

 types, as Dicellograptus, Dicranograptus and Monograptus, they are much 

 more complex. A number of groups can be discerned by the character of the 

 thecae. From these facts they concluded "that the character of the thecae 

 is the most important point to retain in separating families of the Grapto- 

 loidea," and that the next most important point to consider as indicating 

 genetic relationship is the " angle of divergence," while, on the other hand, 

 the number of branches in the rhabdosome, by which our present genera 



