530 NEW YORK STATE MUSEUM 



It has been pointed out by the writer in a former publication [1902, 

 p.586ff] that not only did there exist in the graptolites ontogenetic growth 

 stages in the development of the individual zooids, which, however, can not 

 be traced in the fossils, but the rhabdosomes in toto and their parts, the 

 branches, seem also to pass through stages which suggest phylogenetically 

 preceding forms. These ontogenetic stages of the rhabdosomes express 

 themselves in various ways, of which we select here the direction of the 

 branches and the changing character of the thecae. 



The original direction of the growth of the branches in the Dichograp- 

 tidae has been in the approximate continuation of the sicula, i. e. an ascending 

 erect position as long as the rhabdosomes were sessile on the ground. These 

 became pendent [see ch.6, p.513] when the graptolites attached themselves 

 in a suspended position to seaweeds, as numerous hydroids do today. To 

 restore to the zooids their original, more advantageous, erect position, the 

 branches began now to recurve, a process which in the Dichograptidae led to 

 the Phyllograptidae, the horizontal, the reflexed, reclined and recumbent 

 Tetragraptidae and Didyraograptidae [see ch.lO, p.543], and to the whole class 

 of the Axonophora, where the thecae climb upward along the nemacaulus. 



We find now in the majority of the Dichograptidae with the above cited 

 growth directions of the branches, that the latter still retain their original 

 dependent direction, in the proximal parts in some species, as in Tetra- 

 graptus fruticosus [pl.10, fig.l], Didymograptus nitidus, 

 D. tornquisti and D. patulus, while in others, by the law of 

 acceleration, the dependent proximal direction has already changed into a 

 horizontal one, as in Tetragraptus serra, bigsbyi and taraxa- 

 cum, the change in direction becoming progressively more abrupt as the 

 final direction of the branches becomes reclined, as in T . b i g s b y i , or 

 recumbent, as in Phyllograptus. 



In the Axonophora, where the growth direction of the rhabdosome has 

 become entirely recumbent, it growing upward along the nemacaulus, only 

 the first theca retains for a short distance the original downward direction, 

 and then turns abruptly in a direction at right angles to the former. The 



