538 NEW YORK STATE MUSEUM 



tions. While the two modes of branching produce very different appearances 

 of rhabdosomes, there exists probably no essential difference between thera. 

 In one case the mother theca produces a daughter theca, so early tliat it has 

 not progressed sufficiently to force the latter into a new direction, but both 

 assume new directions, while in the monopodial branching the mother theca 

 has already established its direction, that of the preceding branch, when 

 it sends out a daughter theca, which then alone diverges from the old 

 direction. 



Within the Dichograptidae the angle of divergence changes within the 

 whole compass of the circle, a complete reversion in the direction of the 

 branches gradually taking place. This fact will be noted more fully in the 

 chapter on the classification and phylogeny of the graptolites. 



The structure of the rhabdosomes of the graptolites with diprionid 

 arrangement of the thecae, such as the Diplograptidae and Climacograptidae 

 have, has been little understood till recent jears, but it is now known that, as 

 a rule, and notably in Diplograptus, the rhabdosome is composed of thecae 

 which have assumed a proximal or centi-ipetal direction of growth along the 

 nemacaulus of the sicula, whereby each theca buds from the opposite side of 

 the adjacent more distal one, all arranging themselves thus in two series but 

 actually belonging to one. In certain forms, as in Climacograptus 

 kuckersianus [Wiman, 1888, p.l90], one of the thecae (the third in the 

 species cited) sends out two thecae, and thus two separate series originate 

 which are divided by a longitudinal septum. 



In the Monograptidae which have been derived from the Diplograptidae 

 and Climacograptidae, only one of these series is developed, but the thecae 

 retain their centripetal growth. 



While in the Diplograptidae the tubular thecae have an oblique position 

 to the principal axis of the rhabdosome, thus producing the dentate appear- 

 ance of the latter, they have a rectangular form in Climacograptus and its 

 derivatives, and are appressed in a position parallel to the axis, whereby their 

 outer margins form lines parallel to the axis which are interrupted by the 

 transversal notches of the apertures. 



