534 NEW YORK STATE MUSEUM 



iDecame progressively more rigid and which served to maintain the 

 equilibrium and to give to the greatest number of zooids the most advan- 

 tageous position. 



As, now, the graptolite rhabdosome so closely approached the character 

 of an individual, and like such a one had its ontogeny and repassed 

 ancestral stages, it is possible to recognize such ontogenetic stages of the 

 whole rhabdosome and to apply to these the terminology introduced by 

 Hyatt for the ontogenetic series of an individual. These stages have to 

 some extent been traced in the rhabdosomes of Goniograptus with the 

 following result^ : 



The embryonic stage is clearly present in the initial part of the sicula, 

 which is diiferentiated from the distal part of the sicula by the nature of 

 the periderm, which is thin, pellucid and possesses no growth lines. Holm 

 asserts his belief that this initial, more pointed end of the sicula " corre- 

 sponds to the original chitinous covering of the free zooid germ or embryo." 

 It holds a position similar to the protoconch of the cephalopod shell. 

 The nepionic or infantile stage is represented by forms [pl.'T, flg.1-6] in 

 which the successive dichotomous divisions produce the stems. It begins 

 with the formation of the apertural part of the sicula. The neanic or 

 adolescent stage of the colony begins with the formation of the branches 

 with serial arrangement of thecae and ends, in the Goniograptus material 

 from the Deep kill, with the production of six such branches on each of 

 the four stems. After this, in the ephebic or mature stage, the branches 

 continue to grow out to full length. The gerontic or senile age is marked 

 by a thickening of the stems. 



8 Structure and morphology 



As much of the structure and morphology of the graptolites has been 

 anticipated in preceding chapters, and more details are given in the generic 

 descriptions, but a cursory statement of the principal facts I'egarding 



^ As we have before noted. Professor Cumings has meanwhile proposed to desig- 

 nate the growth pfages of colonies by the prefix " asto." 



