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NEW YORK STATE MUSEUM 



sections of the rhabdosomes are concavo-convex and that this feature is due 

 to the growing of the thecae out of the axial plane of the rhabdosome 

 toward one side (the reverse side) in such a way that in a view of that side 

 [see text fig. 40] the apertures of both rows of the thecae are seen and in 

 the opposite view none. In surveying a large number of specimens of both 

 species, one will find that not all rhabdosomes are equally concavo-convex 

 in their section ; that while some are extremely so, others are merely flat 

 on the concave side and but slightly convex on the other, and that 

 correspondingly the thecae are but little turned toward the reverse side. 



If one looks for an explanation of this peculiar growth direction of the 

 thecae, only one possibility would seem to suggest itself, viz, that this 

 lateral growth is a further modification of the complete 

 reversion in the growth direction of the rhabdosome in the 

 Axonolipa in the effort to attain an ascending position of 

 the thecae. It is hardly to be doubted that these two 

 species grew in synrhabdosomes such as have been found 

 of other Diplograptidae and that probably their synrhabdo- 

 somes bore as many or more rhabdosomes as have been 

 observed in Diplograptus foliaceus, In that case 

 numerous rhabdosomes would in the crowded radiating 

 \ u r s 1 ,] an jp | ? x;cau , 1, , s bundle not find sufficient space to assume vertical or nearlv 



(Hall). Portion of rhab- * J 



se/veTin relief. x 5 pre * vertical positions that would give to the thecae their most 

 advantageous (ascending) direction, but would have to approach a hori- 

 zontal position. It is in the latter rhabdosomes that the thecae would 

 naturally tend to grow out of the axial plane to assume again an upward 

 direction. The facts that the thecae of both rows grew towards the same 

 side and that there exist obvious differences in the amount of this flexion 

 of the thecae in different rhabdosomes can be readily explained by this 

 assumption. 



It is further to be noted that in D. foliaceus, where the thecae all 

 remain in the axial plane of the rhabdosome, we observe extreme differ- 

 ences in the lengths of the free nemacauli. While in the few synrhab- 



