GUAPTOLITES OF NEW YOUK. I'.Mt'r 1 517 



expansions of the thallus of the giant seaweeds, \vhi(;h would be less pliable 

 and fluctuating with the waves. 



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 

 [pl.lO], which would seem to disagree with 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 bands, indicating its hollow character. 

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

 have been filled with gus 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 was also suggested by Ilall ; and the deposition of 

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

 it probably was a hollow body, the filling of which 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 with 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 numbei- of structural 

 departures from the Dichograptidae, which indicate a somewhat diiferent mode 

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

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

 writer has shown [1896, pl.2]. In contrast to the Dichograptidae, the 

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

 by the fact that, with the exception of the synrhabdosomes of Diplograptus 

 found in New 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 have lived in 

 the agitated waters neai- the surface of the ocean. For this reason they 



