GRAPTOLITES OF NEW YORK. TART 1 547 



been made the receptacle of the genera Diplograptus, Glossograptus and 

 Diuiorpliograptus. The family Monograptidi has the same compass as is 

 given to it by Lapworth. The Retiolitidae comprise the genera Retiolites, 

 Gothograptus, Lasiograptus and Clathrograptus. 



We have here adopted with some modification [p.570] Freeh's terms 

 Axonolipa and Axonophora, for the reason that they appear to give proper 

 significance to important differences in the structure of the rhabdosomes. 

 In the former the nema remains free and the thecae are directed distally ; in 

 the latter the viro-ula and nema become the axis of the rhabdosome and the 

 thecae are directed proximally or centrally ; in the former only single 

 I'habdosomes have been observed, while in the latter, as far as complete 

 material has been found, the rhabdosomes were united into synrhabdosomes, 

 and thus a higher and more complicated form of colony attained. As the 

 two orders are also separated in time, the Axonophora replacing the 

 Axonolipa from the middle of the Lower Siluric formation onward, it can 

 hardly be doubted that we have to see in these orders natural divisions 

 of great import. 



The fact that the Phyllograptidae among the Dichograptidae approach 

 by the coalescence of their recumbent branches the Diplograptidae in 

 external appearance, and that, also, the nema appears to become incorpor- 

 ated [see Phyllograptus anna mut. pygmaeus, p.716] into the rhab- 

 dosome as an axis, thus making this form axonophorous to some extent, is 

 liable to lead to the misconception that this family contains forms with 

 some of the distinctive characters of the Axonophora; but the four series 

 of thecae in Phyllograptus represent separate coalescent branches, while 

 in Diplograptus the tw^o series are produced by the alternate arrangement 

 of the thecae of but one series. The axis of the Diplograptidae is the 

 virgula which is already formed in the wall of the sicula, while that of 

 Phyllograptus anna mut. p y g m a e a is only the nema or support- 

 ing thread of the sicula. The axis of the Diplograptidae is hence a new 

 acquisition not found in the Dichograptidae. In some Phyllograptidae, more- 

 over, this thread has been lost or is, as Holm's observations on Phyllo- 



