GRAPTOLITES OF NEW YORK. PART 1 



529 



As the writer lias pointed out in the paper mentioned, the thecae, and 

 with thera the entire rhabdosorae, grow along the nemacaulus toward the 

 central organs, hence in a direction directly opposite to that found in the 

 typical Dichograptidae. The nemacaulus itself lengthens rapidly [^see fig. 9-4 J 



Before the primary rhabdosome has reached mature size, cysts appear 

 around the base of the nemacaulus on the primary disk \^see fig. 9-4], 

 which in older specimens are filled with siculae [^.see fig.9-5]. These 

 cysts, which have been termed gonangia by the writer and considered vesicles, 

 containing the products of sexual reproduction, open finally, a part of the 

 siculae is discharged and form new separate 

 rhabdosomes, while the others remain in con- 

 tact with the primary disk and evolve in the 

 same fashion as the primary rhabdosomes into 

 new rhabdosomes \^see fig.9-7], thus producing 

 the radiating groups of rhabdosomes. Succes- 

 sive generations of gonangia produce whorls of 

 rhabdosomes which differ in length, each whoi'l 

 representing the rhabdosomes grown from a dif- 

 ferent generation of siculae [see fig. 9-8]. 



It will be noticed that the development 

 of the rhabdosomes of the axonophorous Grapto- 

 loidea differs in the following facts from that of the axonolipous Graptoloidea. 

 The nemacaulus of the sicula becomes, by the retrograde growth of the 

 thecae, incorporated into the rhabdosome as a part of the axis, while in the 

 Dendroidea and axonolipous Graptoloidea it remains free. The primary 

 rhabdosome with its one stipe is homologous Avith the entire colony of the 

 Dendroidea and axonolipous Graptoloidea, as it is produced by one sicula. 

 As new rhabdosomes, each originating from a sicula, combine in Diplograptus 

 into a composite colonial stock, this represents a rhabdosome colony or a 

 person of a higher order than that of the Dendroidea and Dichograp- 

 tidae, and the Avriter \_see ch.4, p.483] Las hence proposed for it the term 

 synrhabdosome. 



■Fig. 10 Diplograptus sp. Aper- 



tui-al part of sicula and first theca, 



showing- the perforation from which 



t lie second theca will g-row. x37 (Copy 

 from Wiman) 



