160 ANNALS NEW YORK ACADEMY OF SCIENCES 



lips, another possesses obscure pits and grooves in place of thecal aper- 

 tures ; one group is distinguished by an uniserial, the other by a biserial 

 arrangement of the thecse; a few species belonging to widely separated 

 divisions are proved to be heterothecal, while others are strongly sus- 

 pected to be homceothecal. 



We find similar differences among the genera of Callograptoid ap-: 

 pearance, particularly among the Dendrograpti, the Ptilograpti and in 

 the Galeo-Disco-Cyclograptus group. Those differences are not confined 

 to a certain geological age. Early as well as later species may show the 

 same kind and arrangement of the thecse, while congeners reveal the 

 difference as great as possible in this respect, e. g., the highly variegated 

 Niagaran dendroids of America. This difficulty of a heterogeneric but 

 parallel evolution seems still further increased in complexity by the 

 repeated development of features either retrogressive or intermingled. 

 For we observe the strongly differentiated genus Dendrograptus already 

 at the base of the Ordovician (if not earlier) and yet we see that the 

 path of varietal evolution is in the same direction in much later strata. 

 We observe true Dictyonemas, the neanastic stages of which suggest cer- 

 tain ancestral relationship to members of the Desmograptus or the nor- 

 wegica-murrayi group. That the same is true among the Callograpti is 

 proved by a comparison of Pocta's, Bassler's and Buedemann's species, 

 herein referred to. Even the characteristic bifurcation of the apertural 

 margins and dissepiments has now been found in quite different species 

 and genera, e. g., Ptilograptus, a very doubtful and not unlikely retro- 

 gressive genus, to which some Callograpti described by Poeta bear an 

 undeniable similarity. 



However complicated the paths of evolution of the Dendroidea may 

 be, paths which can be traced back only after the manner of Wiman's 

 and Euedemann's keen-eyed and careful investigations, I am convinced 

 that, generally speaking, they followed directions similar to those mani- 

 fested pre-figuratively in a small but equivalent ratio by the range of 

 variation in Dictyonema flabelliforme. 



