REPORT OF THE STATE PALEONTOLOGIST 1901 585 



thecae of the branches and those of the dichotomously divid- 

 ing stems of Goniograptus which find their expression in the 

 thecae. It may a priori be assumed that innate differences 

 caused the zooids of the stems to assume widely diverging 

 directions and those of the branches to grow in the direction 

 of the mother theca. It is further evident that the essentially 

 or solely nutritive zooids of the branches or stipes persisted in 

 performing the function of nutrition while those of the stems 

 (funicle, etc.) served this function only in the early stages of the 

 colony, and later on, when they became thickened by chitinous 

 deposits into cylindric stems (compare fig. 10, 11 and HalFs 

 figures of Clonograptus rigidus), assumed as their 

 principal or sole function the supporting of the branches. It 

 is partly on account of this secondary adaptation to the latter 

 function that the thecal nature of the stems has failed, till 

 lately, to be recognized in the majority of the dichograptids. 



This difference in function is, to some extent, also expressed 

 in the morphologic differences between the stem thecae which 

 we here call stolonal thecae, 1 and the branch thecae which may 

 be termed brachial thecae. If one compares the extreme 

 thecae of the branches of Goniograptus with those form- 

 ing the stems (fig. 10, 14), one can not fail to notice that they 

 differ. The latter, stolonal thecae, are more cylindric, very 

 slightly widening toward the aperture and without any sub- 

 mucronate apertural processes; they, therefore, usually fail to 

 appear as " denticulations." Their apertures are small, circular 

 openings (fig. 3, 4, 5). The fully developed distant thecae in the 

 branches widen more abruptly toward the aperture, have wider 

 apertures and submucronate processes on the outer apertural 

 margins. These differences can not be due to different degrees 

 of compression in consequence of different thickness of 

 periderm, or be caused solely by the superposition of the thecae 

 on the branches. 



1 Tlie first theca of each "denticulate" branch is to be considered as 

 a stolonal theca on account of its assuming a direction different from 

 that of the mother theca. 



