. Annual report. New York State Museum; Science; Science. GRAPTOLITES OF NEW YORK, PART 2 103 But even if such axes as depicted by Freeh existed in Dicellograptus, they were in no way homologous to those of Climacograptus or Diplo- graptus, for the virgula of a Diplograptus is initiated in the wall of the sicula and grows in antisicular direction within the nemacaulus. In Dicel- lograptus, however, the apical end of the sicula lies outside of the branches, and its short, stiff nema can frequently be seen protruding freely into the axillary angle \_see fig. 42]. It could, hence, not continue in
. Annual report. New York State Museum; Science; Science. GRAPTOLITES OF NEW YORK, PART 2 103 But even if such axes as depicted by Freeh existed in Dicellograptus, they were in no way homologous to those of Climacograptus or Diplo- graptus, for the virgula of a Diplograptus is initiated in the wall of the sicula and grows in antisicular direction within the nemacaulus. In Dicel- lograptus, however, the apical end of the sicula lies outside of the branches, and its short, stiff nema can frequently be seen protruding freely into the axillary angle \_see fig. 42]. It could, hence, not continue into the branches of a Dicellograptus, but may become incorporated, as we have seen above, into the biserial portion of a Dicranograptus. The virgula of a Dicello- graptus would, therefore, have to be a new acquisition, different from that of a Climacograptus. As such it could be assumed to be a continuation of a supposed virgula of the sicula in opposite or apertural direction, the virgula dividing into two branches, but the presence of a virgella, which is the legitimate continuation of the virgula in t/f g4«ri?yicelLaPgJort^ Sicular portion of rhabdosome that direction, opposes such assumption. preserved in relief. *7 There are also phylogenetic considerations militating against the con- nection of Dicranograptus and Dicellograptus with the Axonophora. There is excellent reason for deriving Dicellograptus from Leptograptus [see p. 116] and Dicranograptus from Dicellograptus. Leptograptus, however, like all Dichograptidae does not possess any axes (nema and virgula) within its branches, the nema always remaining independent of the branches. The general structure of the rhabdosome has not changed materially in the development of Dicellograptus from Leptograptus, the differential characters lying mainly in the form of the thecae. There is, hence, no reason for the development of axes in Dicellograptus. The formation of the virgula in the Diplograptidae and Climacograp- tidae evi
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