570 NEW YORK STATE MUSEUM 



of the colony. It was hence of advantage to shorten the branches in the 

 forms where they are reclined. For this reason we find the broad, short 

 branches in Tet rag rapt us (bigsbyi) similis and Didymo- 

 graptus (gibberuliis) caduceus, species in which, and in the 

 Phyllograptidae, the reclining has gone the farthest. 



e Classification adopted here. We have shown in the preceding dis- 

 cussion that the phylogenetic relations of the principal groups are not yet 

 elucidated, while those of the families and genera as far as known indicate 

 the necessity of a future revision of the present more or less artificial 

 groupings. It would, under these circumstances, be premature to attempt 

 new classifications ; and we use here that still in general use and based on 

 Lap worth's Improved Classification of the Rhahdophora, with such modi- 

 fications as are suggested by later investigations, and which we cite here 

 briefly. 



Lap worth comprised under the Rhabdophora all graptolites with the 

 exception of the dendroid forms, at that time still doubtfully referred to 

 the graptolites. For the latter the term Dendroidea proposed by Nicholson 

 is currently used. The Rhabdophora were divided by Lapworth into the 

 Graptolitidae and the Retioloidea. 



As is indicated by the similarity of the thecae of various Retioloidea 

 to those of forms of Diplograptus and Climacograptus, and by the early 

 appearance, in Retiograptus, but late acmic development of the group as a 

 whole (after that of the Diplograptidae proper), this group is of polyphyletic 

 origin and composed of terminals of various series which have adapted 

 themselves in a parallel manner to the floating habit of the graptolites 

 [p.518]. We have, for this reason, not recognized here the order Retioloidea 

 but united the single representative in our fauna (Retiograptus) with the 

 Climacograptidae. 



Freeh has divided the graptolites into two orders, the Axonolipa 

 and Axonophora, according to the absence or presence of an axis. While 

 we have noted before [p. 5 5 2] that this axis may not in all genera be a 

 homologous organ, and hence its recognition as a diagnostic character is 



