GRAPTOLITES OF NEW YORK, PART 1 549 



tions of the internal structure of Graptoloidea, can not be considered as 

 constituting a fundamental difference between tbe two orders. 



Elles [1898, p.536] also places, in a table giving the "suggested phy- 

 logeny of a part of the Skiddaw slate graptolites," Dictyonema at the base of 

 the system leading through Bryograptus to Tetragraptus and Didymograptus. 



If we add that the sudden appearance of the progenitors of the Dicho- 

 graptidae, the genera Brj^ograptus and Clonograptus, in the wake of the 

 equally sudden and widespread appearance of Dictyonema f 1 a b e 1 1 i - 

 forme may be due, as Lapworth has suggested, to the change of these 

 forms from a sessile benthonic mode of life to a planktonic or pseudoplank- 

 tonic one, the conclusion appears legitimate that the Dendi'oidea precede the 

 Graptoloidea in time, and that the possible transitional forms between the 

 two orders, being still benthonic, are not readily accessible to observation ; 

 that, hence, the failure to observe them is no argument against their former 

 existence. 



It may be further stated that the possession of like initial receptacles, 

 the siculae, and the similaritj- of the early parts of the rhabdosomes in 

 Dictyonema flabelliforme and Staurograptus [pi. 2 J can also be 

 regarded as suggesting common ancestors for these, and a consequent closer 

 relationship, as the mature colonies would indicate. 



As an alternative theory, the possibility of a derivation of the Den- 

 droidea from the Graptoloidea has been pointed out by Wiman ; this 

 phylogenetic relation being considered by that author the more probable on 

 the ground that differentiation of individuals frequently is produced by a 

 division of labor. This theory, it is thought, might also explain the fact that 

 the proximal thecae of Dictyonema peltatum are similar to those of 

 Graptoloidea. 



The solution to the entire problem of the relations between Dendroidea 

 and Graptoloidea rests obviously with the discovery of earlier Cambric 

 graptolites, and specially with that of the structure of the reproductive 

 individuals in the Dichograptidae. 



