488 NEW YORK STATE MUSEUM 



stage coming under notice. The disk forming the center of the synrhabdosome 

 of Diplograptus is a further development of this primary disk and should, 

 hence, not have been termed a " central disk," as was done on a former occasion 

 by the writer. 



Basal cyst or pneumatocyst has been applied to a tenuous chitinous 

 vesicle, observed in Diplograptus by the writer, which is supposed to have 

 served as a " float." 



Dendroidea Nicholson. This term embraces the dendroid graptolites. 



Graptoloidea Lapworth. These are the graptolites proper. 



Retioloidea Lapworth. In this group the periderm consists of a network 

 of chitinous filaments. 



Axonolipa Freeh. In this order the virgula or solid axis is lacking. 



Axonophora Freeh. The forms ot this order are provided with a virgula. 



5 Range and geographic distribution 



The graptolites begin with a few undoubted forms in the closing stage of 

 the Cambric, attain their acme in the Champlainic (Lower Siluric), reach a 

 second, minor period of prosperity with a small group of newly developed 

 genera in the lower and middle Siluric and disappear mth astonishing 

 rapidity in the Upper Siluric, leaving but a few stragglers (four species in 

 North America) in the Devonic. ^ For this reason they can be considered a 

 class of organisms most characteristic of the Champlainic and Siluric eras. 



During the period of their greatest vitality they attained a remarkable 

 rapidity of development, which has permitted the division of the graptolite 

 shales of several regions into a great number of life zones. This is notably 

 the case in certain parts of Sweden, where, for instance, TuUberg cites from 

 Scania [1882] no less than 26 graptolite zones of the Lower and Upper Siluric. 

 This fact, together with the discovery brought out by investigations in 

 Great Britain, France, America and Australia, that the larger divisions 



^Gurley has also described a fossil from the Choteau limestone (Carboniferous) of 

 Sedalia Mo. as Dictyonema blairi. This, however, is very doubtful and according 

 to last accounts only the remains of a plant. 



