GRAPTOLITES OF NKW YORK. PART 1 513 



represent a zone between the agitated bottom where coarser sediments are 

 deposited and the dead water of the deep sea. 



Lapworth infers further, from the writer's observations on Diplograptus 

 and other facts, that the nema of every sicnla Avas originally an organ of attach- 

 ment, either to a " central organ " or to a foreign body. He has found, that in 

 the Dendroidea, to which also Dictyonema belongs, all transitions occur 

 from the stem of the typical Dendroidea to the threadlike nema. In the lat- 

 ter case they must have been suspended like a bell at the end of a rope from 

 the supporting object. They were now, according to Lapworth, attached to 

 floating objects, as seaweeds ; and this is held to explain both their constant 

 appearance in carbonaceous sediments and their world-wide distribution. In 

 support of this hypothesis are further cited the facts that only the siculate and 

 funiculate graptolites -swarmed in such multitudes over the world, while the 

 sessile Dendroidea are by no means common ; that further the first profuse 

 appearance of graptolites, in the upper Cambric Dictyonema fauna, is that of 

 siculate suspended forms. These, Lapworth argues, are the first types in 

 Avhich the originally l3euthonic mode of existence has changed into a pseudo- 

 plauktonic ; and it is suggested that they thus escaped from some creeping 

 enemy. 



With this change in the mode of existence was connected the complete 

 reversion in the position of the whole rhabdosome which gradually took place 

 during the development of the various graptolite series.^ Its purpose was to 

 restore to the thecae their original ascending direction. 



The distinguished author concludes by stating that it can be considered 

 as established, that some graptolites, specially the Dendroidea, were attached 

 during their lifetime to fixed objects, that possibly others belonged to the 

 plankton proper, but that the majority of the typical graptolites lived as 

 pseudoplankton. 



^ This interesting and significant reversion is fully described in the chapter on the 

 phylogeny of the graptolites [see p.530]. 



