GUAI'TOLITES OF NEW YOUK. rAlt'l' 1 517 



expaiLsious of the tliallu.s of the giant seaweeds, Avlncli \voiild be less pliable 

 and fluctuating with the ^vaves. 



In this connection the thick nema of T e t r a g r a p t u s f r u t i c o s u s 

 [pi. 10], Avhich would seem to disagree Avith the foregoing remarks, deserves 

 special notice. Complete specimens show however that this nema tapers 

 upward into a fine thread, and that hence also these beautiful rhabdosomes 

 were suspended, as is clearly indicated by the recurving of the branches. 

 Moreover, in several cases the compression of the organisms has separated 

 this apparently thick stem into two bauds, indicating its hollow character. 

 It suggests itself readily that this secondary expansion of the nema may 

 have been filled with gas and to some extent aided in supporting the 

 large and heavy rhabdosome. The later developed central disk of some 

 Dichograptidae appears from the writer's observation [p.746] to have been 

 composed of two layers, as w^as also suggested by Hall; and the deposition of 

 lime betw^een its walls described in this paper | p. 74(5] would indicate that 

 it probably was a hollow body, the filling of Avhich with gas may, at 

 times of accidental separation from the supporting seaweeds, have saved 

 the rhabdosome from sinking to the bottom. 



After the development of the Dichograptidae, and Avith them that of the 

 axonolipous forms, has reached its acme in the second Deep kill zone, the 

 axonophorous forms, represented by Diplograptidae and Climacograptidae, 

 appear in the third zone. In these we find a number of structural 

 departures from the Dichograptidae, which indicate a somewhat different mode 

 of existence. These are the presence of a solid axis, Avhich already originates 

 in the Avail of the sicula and extends into the nemacaulus, as the present 

 Avriter has shoAvn [1896, pl.2]. In contrast to the Dichograptidae, the 

 mostly long nemacaulus is straight, stiff and fragile, as clearly evinced 

 by the fact that, Avith the exception of the sjmrhabdosomes of Diplograptus 

 found in NeAV York, nothing but broken off rhabdosomes of these immensely 

 common graptolites have been found. This character of the nemacaulus 

 is incompatible Avith the assumption that these forms could ]iave lived in 

 the agitated Avaters near the surface of the ocean. For this reason they 



