514 NEW YORK STATE MUSEUM 



Freeh [Lethaea palaeozoica, 1 : 684] considers the graptolites as gre- 

 garious, free planktonic organisms. His conception is based on the present 

 writer's observations on Diplograptus, while Lapworth's views, just cited, had 

 not been published and have therefore not been taken into consideration. Of 

 special interest is Freeh's discussion of certain hitherto problematic features 

 of the graptolite structure in the light of a possible planktonic existence 

 \ibid. p.552]. The semicircular expansions of the distal ends of the rhabdo- 

 somes of Diplograptus physophora, formerly described by Lap- 

 worth as air cells, the distal " discus " of Monograptus pala and the 

 well known distal expansions between strong spines of Climacograptus 

 bicornis are considered as steering rudders used in the rising and sinking 

 of the synrhabdosomes. The rudderlike broadening of the entire stipe 

 of the later Diplograptidae (Cephalograptus) and the alate pellicle of 

 Dicellograptus divaricatus var. rigida^ are also held to have 

 subserved the same function. The solid axis of the rhabdosomes, in which 

 "Wiman saw an argument against their possible suspension, is explained as 

 having served to strengthen the rudder pole. 



In undertaking to present our own views on this important problem of the 

 mode of existence of the graptolites, we will state at the outset that our 

 observations agree fully with the facts brought forward by Lapworth, and 

 that we believe his hypothesis furnishes a very plausible explanation for 

 numerous characters of at least one large group of graptolites. It appears 

 further that, in the discussion of the problem before us, the three principal 

 groups of this class, which succeed each other in time of appearance, 

 namely the Dendroidea, the Axonolipa or the Graptoloidea without solid 

 axes and the Axonophora or forms with axes [p. 546], should be treated 

 separately. 



The Dendroidea approach in their habit most closely to certain recent 

 Hydrozoa ; they have been observed to possess sometimes a distinct network 



'The New York State Museum contains specimens belonging to this genus in which . 

 a continuous pellicle is stretched between the entire distal part of the two branches. 



