GRAPTOLITES OF NEW YORK, PART 2 73 



first two thecae develop one gigantic spur each that is placed on the ventral 

 or outer side of the theca [see text fig. 6]. In Cryptograptus tri- 

 cornis [see text fig. 7] there are four spines developed from the apertural 

 rimr of the sicula. 



All of these above cited observations point to causal connection 

 between the spines and the exposure to external stimuli of the places 

 where they grow. The form and strength of the spines further leave no 

 doubt that they have secondarily become adapted to organs of defense and 

 thereby attained considerable importance. In the case of Climaco- 

 graptus bicornis, which will be treated at the end of this chapter, a 

 further development and adaptation to an entirely different function has 

 taken place. 



Besides the cases of spiniferous forms cited thus far, there are others 

 which can not be explained by the influence of external stimuli and still 

 others which present features that point to the agency of additional factors. 

 Most prominent among these are the cases of strong spinosity in the last 

 dwarfed phylogerontic mutations of otherwise nonspinose forms, already 

 noted in part 1 in the descriptions of Tetragraptus pygmaeus, 

 Didymograptus caduceus var. nanus and Phyllograptus 

 anna mut. u 1 1 i m u s. Here also belongs Didymograptus nanus, 

 which is figured as a spinose form by Perner and others. In regard to 

 Didymograptus caduceus mut. n a n u s I had stated [pt 1, 

 p. 698] that " its constantly smaller size, its wider proximal part, the abrupt 

 narrowing of the branches, the smaller size, and closer arrangement of the 

 thecae and the spinose processes of the lower ends of the apertural margins 

 indicate a concentration of the entire development of the rhabdosomes 

 into smaller space and shorter time, denoting the paracmic condition of 

 the mutation." All of these cited dwarfed forms appear at the very end of 

 their " phyla " and are undoubtedly the last weak survivals of their races. 



The supposition that they might have degenerated by unfavorable 

 physical conditions or "restraint of environment" is contradicted by their 

 position at the end of the race and still more by the fact that in the 



