552 NEW YORK STATE MUSEUM 



this coenogeuetic character iu the sicula is also evidence of the extreme 

 rapidity of develoj)meiit among the graptolites, evinced already by the rapid 

 changing of the faunas in the successive beds. 



It is, however, in my opinion, doubtful whether the solid axis or virgula 

 of the Axonophora is a homologous organ in the different groups of that 

 order. It seems almost impossible that the virgula of Diplograptus, which 

 originates in the sicula and nemacaulus, and with the latter becomes incorpo- 

 I'ated in the rhabdosome, should be homologous to the bipartite axis found 

 within the two reclined branches of Dicellograptus ; for the nema from which 

 the sicula of the latter Avas originally suspended, is, according to my observa- 

 tion and knowledge, never incorporated in either of the branches. The axes 

 of Dicellograptus appear to be, for this reason, only thickenings of the 

 dorsal wall of the coenosarcal canal, induced by the upward growth of the 

 branches, while the A'irgula of the typical Axonophora is a separate rod. Such 

 secondary strengthening of the branches is described in this memoir even 

 from a dichograptid, viz Tetragraptus amii [p.647 and pl-.l 1 J. 



The early appearance of a Dicellograptus, viz D . m o f f a t e n s i s , in 

 the zone with Diplograptus dentatus of the Upper Skiddaw slates, 

 is very suggestive of the derivation of that genus, not from the later Dicrano- 

 graptus, as generally supposed, but from Didymograptus. From the latter it 

 is only separated by the presence of the solid virgula and the peculiar shape 

 of the thecae. Both are probably secondary acquisitions, and the form, 

 described here as Didymograptus i n c e r t u s , appears to me to indi- 

 cate the path of this derivation ; for, though it is in all its appearance and 

 by the presence of the dorsal thickening a Dicellograptus, it has the thecae of 

 a Didymograptus. As the peculiar thecal shapes of the Upper Champlainic 

 and Upper Siluric species are distinctly .later acquisitions, appearing only 

 toward the end of the graptolite reign, they are here of no phylogenetic 

 significance. 



If then the virgula of Diplograptus is an organ originating within the 

 sicula and nemacaulus, and that of Dicellograptus an organ which originates 

 within the Avails of the coenosarcal canal of the branches, they can not be 



