GRAPTOHTES OF NEW YORK, PAllT 1 535 



those groups of graptolites which are treated in this memoir is needed in 

 this place. 



All graptolites are colonies of hydroid-like appearance. Only the car- 

 bonaceous (chitiuous) periderm is preserved. 



Probably all colonies originated from an embryonal zooid (sicula) and 

 consisted, where complete, of (1) the organs of attachment, or suspension 

 (2) the supporting stems, (3) the thecae, and (4) the reproductive organs. 



The organ of attachment may be a part of the first embryonal 

 zooid (primary disk) or a secondary disk (central disk) or a rootlike 

 expansion. The first organ is probably found in the young stages of all 

 graptolites, and in the mature stage of all smaller Axonolipa and probably 

 in all Axonophora; the second in some larger forms of the Dichograptidae 

 (genera Tetragraptus and Dichograptus). Perhaps it occurs also in some 

 species of Didymograptus, as in D. p a t u 1 u s . The rootlike expansions 

 have been observed only in some Dendroidea (see Dictyonema).^ 



Between the sicula and the primary disk is intercalated either a thin, 

 often very long, filament, the nema, in the young of some Dendroidea 

 (Dictyonema flabelliforme) and the Axonolipa, or a more rigid, 

 narrow tube (nemacaulus) in the Axonophora, which in the latter is 

 (always?) supported by a strengthening rod (virgula)* originating in the 

 wall of the sicula. 



From the sicula originates the first theca, and by continued gemmation 

 of thecae the branches are formed. The second theca in nearly all Grapto- 



' In some Axonophora a vesicle has been found by the writer to surmount the 

 primary disk. This is considered to have had the function of a pneumatophor. 



' The inclosure of the virgula within the nemacaulus of the Diplograptidae can be 

 inferred from an observation made by the writer [1895], who found in a flexed specimen, 

 that the virgula had separated from the nemacaulus \loc. cit. pl.2, fig.6]. Tlie latter 

 is, according to Wiman, who observed its initial part, a hollow tube. Sometimes 

 the nemacaulus becomes inflated into a vesicle, as in Dipl. appendiculatns 

 (Tornq. ras.) EUes emend. Inside this vesicle the virgula can, according to Elles [1898J 

 be quite well detected ; and it is also figured \iUd. fig. 30] by that authoress as a straight 

 rod extending through the vesicle. 



