GRAPTOLITES OF NEW YORK, PART 1 563 



difficulty be distinguislied from a Bryograptus [p.GlG and pl.2j suggest to 

 us the inference that Bryograptus is derived from Clonograptus by a 

 suppression of half of the primary dichotomies and a retention of the 

 irregular branching. Moreover, the difference between the raultiramous 

 Dictyonema and the proximally biramous Bryograptus is still too great 

 to warrant the assumption of a direct derivation without the intercalation 

 of forms such as Clonograptus with fewer branches than Dictyonema has 

 and more than Bryograptus has. 



The great similarity in the exterior aspects of the proximal portions 

 of the rhabdosomes of Clonograptus (Staurograptus) dicho- 

 t o m u s and Dictyonema flabelliformeis pointed out under the des- 

 cription of these forms [^see also pl.l, fig.15; pl.2, fig.9]. This is due to the 

 similar initial branching and may be entirely accidental, but is certainly quite 

 suggestive of a common ancestor in view of the presence of these two 

 forms alone in the lowest graptolite bed. 



For these reasons, we incline to the view that all Dichograptidae will 

 be finally traced back to Clonograptus forms, from which one part develops 

 through Bryograptus, the other through Loganograptus and Dichograptus, 

 into Tetragraptus and Didymograptus stages. 



The Phyllograptidae have, by Holm's investigation, been shown to be 

 Tetragrapti of the recumbent series, the branches of which have coalesced 

 with their dorsal sides. A transitional form, in which the proximal parts 

 of the four branches also are said to be united, is reported under the name 

 Tetragraptus phyllograptoides (Linnarsson ms) from Sweden 

 \8ee Elles, p.534]. This would seem to lead from Tetragraptus bigsbyi 

 to Phyllograptus angustifolius. Whether the other species 

 of Phyllograptus branched off from this radical form of the group, or 

 whether also the Phyllograptidae are of polyphyletic origin, has not yet 

 been investigated. At any rate, no forms of Tetragraptus are known to 

 which the other species of Phyllograptus, P. ilicifolius, P. typus 

 and P . anna, could be readily referred. There seems to have taken 

 place, not only a condensation of the branches and a resulting curving 



