GRAPTOLITES OF NEW YORK, PART 2 IOX 



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 Dicell >- 

 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 ? s g ' g 4 Srieyi ce La/wor& 



Sicular portion of rhabdosome 



that direction, opposes such assumption. preserved in relief. x 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.i 16] 

 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 evidently results from several causes, viz, the biserial growth of the 

 rhabdosomes, their suspension from a nemacaulus and the possible function 

 of the former as steering apparatus. 



It is in this connection quite interesting to note that where a biserial 

 portion is formed, as in Dicranograptus, this is found to incorporate the 



