GRAPTOLITES OF NEW YORK, PART 2 



I05 



It seems hence that there are two or more factors operative in produc- 

 ing the curvature of the branches. One of these is the tendency in the 

 suspended rhabdosome to give the branches a recurving form and thereby 

 to gain for the thecae an ascending direction. This tendency and its phy- 



Fig. 43,44 Didy mograptus subtenuis (Hall). Proximal and distal portions of rhabdosomes preserved 

 in relief, x 7 



Fig. 45 D icell ogra pt us gurleyi Lap worth. Rhabdosome preserved in relief, x 7 



logenetic aspects have been fully discussed in part 1. Another is the 

 influence, just mentioned, of the aperture of the mother zooid on the growth 

 direction of the daughter zooid. 



It is quite probable that in the initial divergence of the young theca 

 an ontogenetic character of some phylogenetic bearing has to be seen, and 



