518 NEW YORK STATE MUSEUM 



must have lived either in quiet bottom Avaters, as Wiman has urged on 

 account of this stiffness of the nemacaulus, or have been floating in some- 

 vphat deeper and quieter waters than the Axonolipa. The latter alternative 

 presents, on account of the reversed direction of growth of the thecae, the 

 most probable hypothesis. It is supported by other facts and considerations. 

 The character of the central vesiclelike body in Diplograptus described 

 before as pneumatophor by the present writer [1895], the presence of 

 the rudderlike expansions of the distal ends of the rhabdosomes in several 

 species and the bladelike form of the entire rhabdosomes of others, would 

 suggest the adaptation of the synrhabdosomes to a vertical and oblique 

 rising and sinking. 



Among the Axonophora a strong tendency to a lightening of the 

 rhabdosomes by extensive perforation of the peridermal walls, which finally 

 leads to a perfect reticulation, makes itself felt, as well among probable 

 descendants of the Diplograptidae as among those of the Climacograptidae, 

 and leads to the production of the genera Retiolites, Stomatograptus, Gotho- 

 graptus, Lasiograptus, Clathrograptus and Retiograptus. The purpose of 

 this tendency can not be well understood, if either an attachment to seaweeds 

 or to the bottom is assumed, since in either case only a weakening of the 

 protective covering without apparent accruing advantage would be attained ; 

 but it is readily referred to a planktonic mode of existence, as all planktonic 

 forms tend to lighten their shells. 



It might be further mentioned that in certain beds of the Utica shale of 

 the Mohawk valley fragments of seaweeds are by no means rare and are 

 often covered with such organisms as the sessile Conularias, described by. the, 

 writer, but that in no case have colonies of the axonophorous graptolites 

 been found attached to them; the graptolites occurring in these beds are, 

 on the contrary, only represented by detached rhabdosomes. 



Another argument for the free planktonic mode of life of the Diplo- 

 graptidae at least, is seen by the present writer in the observation published 

 by him before [1895], that the siculae, discharged from the gonangia are 

 already provided with their pneumatophors while in the Hydroidea, where 



