104 NEW YORK STATE MUSEUM 



nema of the sicula thereby producing in the Dicranograptidae a homolo^ue 

 of the axis of the Diplograptidae ; and this short piece of nema is the only 

 representative of an axis in that group of which we are aware. In basins 

 the reference of Dicranograptus to the Axonophora on this short nema, one 

 would have to consider the whole group of the Dicranograptidae as a 

 transitional one between the Axonolipa and Axonophora or rather, since 

 Dicranograptus appears to become extinct, as a group which has not fully 

 reached the level of the Axonophora. 



i On the morphology of the thecae of the Dichograptidae and Dicranograptidae 



There occur in the Normanskill shale of Mount Moreno near Hudson 

 pyritized specimens of Dichograptidae and Dicranograptidae which shed 

 some interesting light on the morphology of the thecae and branches. In 

 these specimens the somatic cavities are filled with pyrite while the peri- 

 derm has either been entirely destroyed or reduced to a mere coloring film. 

 As a result of this favorable preservation, certain features of the interior 

 structure are revealed, no indications of which are detected in external 

 views. 



We select here for description specimens of D i d y m o gr ap t u s 

 subtenuis and D i c e 1 1 o g r a p t u s g u r 1 e y i. 



Text figures ^3 and 44 represent proximal and distal portions of a 

 rhabdosome of the former species. The sicula and primary thecae are not 

 well shown. The internal casts of the proximal part of the branch demon- 

 strate the following morphologic facts. The new theca diverges from the 

 mother theca in its growth direction for a brief period but returns imme- 

 diately to it thus producing a small cavity just beyond its budding point 

 (best seen in the mature thecae, figure 44). It then adheres to the mother 

 theca as far as the aperture of the latter where it again diverges though 

 but very slightly. This new direction is then retained. It is that of the 

 branch for the length of the remaining part of the theca and becomes thus 

 the factor that produces the gentle curvature of the proximal part of the 

 branch. Without it the branch would remain straight. 



