GRAPTOLITBS OF NEW YORK, PART 1 661 



parts of the thecae which have developed later. There appears also to be a 

 crossing canal (or something of a similar nature) between th. 2* and th, 1*. 

 Thus, as Tornquist points out [loc. cit.'] 'The first stipe crosses the sicula 

 and the second stipe the first theca.' This seems to show that in this species 

 there is a deviation from the normal Didymograptus type of development, 

 that is to say, a forecast of the type characteristic of the Diplograptidae." 



If, indeed, the budding of thecae on alternate sides, which produces the 

 double intertwined series of thecae in Diplograptus, originated with the first 

 thecae of a Didymograptus with reclined branches, such as D. gibberulus, 

 the acceleration of the development of the ancestral characters must have 

 been extremely rapid, for the only vestige left of the former downward 

 direction of the first thecae of the ancestral Dichograptidae is the short, 

 initial, downward course of the first theca in Diplograptus and Climaco- 

 graptus [text fig. 10]. This and the downward direction of the sicula seem 

 indeed to be the only facts of ontogenetic importance for a phylogenetic 

 linking of the Axonophora with preceding graptolites. The fact of the 

 replacement in time of the Dichograptidae by the Axonophora would seem to 

 lend support to the assumption of this phylogenetic connection of the two 

 orders of graptolites. 



On the other hand, it should not be forgotten that the siculate rhabdo- 

 somes of the Axonophora unite into individuals of a higher order, the 

 synrhabdosomes of Diplograptus and Retiolites, and thus an additional 

 distinctive character is found in the Axonophora no indication of which 

 is as yet known among the Dichograptidae. 



The incorporation of the nemacaulus in the rhabdosome of the Diplo- 

 graptidae as support of the backward or upward growing thecae, which, so to 

 say, climb upward along the nemacaulus, has induced the formation of a 

 special organ among the Axonophora, the virgula, which is found to originate 

 within the wall of the sicula of these forms, as far as they have been studied. 

 This appears, then, to be an interesting case of the transference by 

 tachygenesis of a character, the virgula, newly acquired by a colony, to 

 the embryonic stage (sicula) of the \vhole colony. The appearance of 



