GRAPTOLITES OF NEW YORK. PART 1 



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branches pass lieuce, in their development, through dif- 

 ferent directions representing ontogenetic stages that 

 repeat stations in their phylogenetic development. 



An analogous fact is found in the character of the 

 thecae. The change Avhich takes place in the character of 

 the thecae within the colonies of the Dichograptidae has 

 been described in detail in the paper noted before, on the 

 development of Goniograptus thureaui [1902, 

 p.586]. The writer has endeavored to demonstrate that 

 in the I'habdosomes of that species two different kinds 

 of thecae can be recognized, (1) those in the more 

 proximal portions forming the bifurcating branches, for 

 which the term stolonal thecae was proposed, and (2) 

 those in the distal parts, notably in the serially arranged 

 jSnal branches. These were termed brachial thecae. The 

 stolonal thecae are more cylindric than the brachial 

 ones, remain narrowei' toward the aperture, less inclined 

 to the axis of the branches and without any submucro- 

 nate apertural processes. 



A comparison of the form of the thecae of the 

 younger dichograptid genera, as of Dichograptus, Tetra- 

 graptus and Didymograptus, with that of the older and 

 presumably phylogenetically preceding genera, Bryograp- 

 tus and Clonograptus, shows that in general the older 

 genera have the more tubular, simpler thecae with less 

 protected apertural margins. It is, hence, apparent that 

 the stolonal or earlier thecae of the rhabdosomes repre- 

 sent indeed the older types of thecal form. 



Observation of the thecae constituting the " denticu- 

 late " or final branches of G o n i o g r aptus thureaui, 

 of Tetragraptns fruticosus [.see fig. 11], and of 

 T. taraxacum has further brou2;ht out the fact that 



Fig. 11 Tetragrap. 

 tus fruticosus Hall. 

 Branch which shows pro- 

 gressive change of thecae. 

 x-i>t (Copy from Ruede- 

 man) 



