HAHN, DICTYONEMA-FAUNA OF NAVY ISLAND, N. B. 157 



evolution and that it represents one of the usual lines of development of 

 such benthonic forms under a given environment. 



Now, with all the foregoing statements, we may touch upon the final 

 question of the phytogeny of the dendroids as indicated by the range of 

 variation in the earliest Dictyonema. A certain branch of the planctonic 

 .ancestors common to both Graptolites and Dendroidea was evidently 

 pushed forward in the lines of directed evolution by the formation of 

 dissepiments as a supporting mechanism of the elongating rhabdomes. 

 This represents the Dictyonema stage, beyond which a group of retarded 

 species and genera (e. g., Desmograptus) never passed to any consider- 

 able extent. While in the Dictyonemas of the early days, fixation was 

 realized only by means of a thin, fragile nema, the adhesive organs were 

 now brought into vital and ever increasing significance. For gradually 

 thickened stems with basal expansions, with stolonial ramifications, with 

 ability of independent budding of thecse and of colonies, were built, 

 while the original planctonic period of life becomes shortened to its final 

 disappearance (Dendrograptus-st&ge) . From this point on, an extreme 

 widening of the main stipe, on one side, gave rise to the Galeograptus, 

 Discograptus, Cyclograptus, Bodanograptus-gTOivp, while a thickening 

 of the central axis led to forms like Inocaulis, Acanthograptus, Cacto- 

 graptus, Palcedictyota, to which even Chaunograptus, Corynoides and 

 Thamnograptus may be related, as held by Euedemann. Finally, in types 

 like Mastigograptus, a striking approach to the present hydroids has been 

 revealed. When considering the various races of Dendroidea on such a 

 broad basis, the various genera do not of course mean an} r thing else than 

 .stages in development; and every line of separation seems an arbitrary 

 •one, as is shown in a comparison of the species thus far assigned to 

 "one genus" by the different authors (e. g., the Callograpti and Dendro- 

 grapti). Nor is the difference between the forms in the early Ordovician 

 with free siculse and those with unknown siculas a reliable one upon 

 which alone to base the natural classification. 



Within the last five years, two papers of such importance regarding 

 "the differentiation and evolution of the Dictyonemas have been published 

 that they must be considered with great care. In 1907, W. S. Fearnside 

 (3) made some striking suggestions. He believes that 



""in the earliest Dictyonema, the cells are very indistinct and rarely project 

 more than about a quarter of the diameter of the common canal ; cross-threads 

 thin and numerous ; stipes close together, parallel, branching at all levels ; 

 -elongated rectangles of meshes. This supposed Dictyonema diverges in two 

 distinct families, one approaching the true graptolites (Dictyograptas), the 

 •other seems more nearly related to the Dendroids (Dictyonema, sensu stricto). 



