GRAPTOLITES OF NEW YORK, PART 2 §7 



naturally increase materially the weight of the rhabdosome and thus call for 

 the reduction of the continuous periderm. 



It is not claimed that all retioloid structures have originated from the 

 formation of spines, for some have none. Spinosity is considered as but 

 one of the factors of the production of the Retioloidea. Another one has 

 been pointed out in Memoir y, page 518, where the extensive perforation of 

 the periderm in retioloid forms has been ascribed to a general tendency 

 to lighten the rhabdosomes induced by the assumption of holo- and pseudo- 

 planktonic modes of life in the later graptolites. 



The question is also here left open whether the retioloid layer in the 

 spinose forms is lying below the continuous layer of the periderm or is only 

 a constituent part of the same in form of a strengthening framework, which 

 finally alone remains. The latter supposition is perhaps nearer the truth. 

 It is indicated by Wiman's observation that in Gothograptus n a s s a 

 the meshes are sometimes connected by fragments of a membrane, and 

 Wiman directly terms the meshes the strengthening supports of the middle 

 layer. On the other hand, this retioloid framework does nowhere in the 

 spinous forms here discussed project on the surface of the rhabdosome, 

 which is smooth ; and it must therefore, if a part of the middle layer of the 

 periderm, have extended inward and formed, so to say, an inner substratum. 

 For this reason we have here conveniently termed it an inner portion of the 

 middle layer of the periderm. 



d Dilatations ("vesicles") of nemacaulus in Diplograptus, Climacograptus and 



Cryptograptus 



In several species of Climacograptus and Cryptograptus described in 

 this memoir, the nemacaulus exhibits peculiar expansions which in at least 

 two species attain such prominence that at times they must have been of 

 considerable functional importance. 1 



'These expansions are strictly to be separated from those found at the other, sicular 

 end, of the rhabdosome in such forms as Climacograptus bicornis var. pel- 

 t i f e r . The latter originate from the virgella and the lateral spines and the former from 

 the stem or nemacaulus. Nor do they seem to have had like functions. 



