CIJAPTOLITES OF XEW Y.OUK. PAUT 1 705 



Tliis 1-elationship has ncnv l)eeii fully demonstrated by Holm's funda- 

 mental ^vork on the genei'a Didymograptus, Tetragraptus and Phyllograptiis 

 [189;"), p.27]. Holm isolated specimens of P. an gu s t if o 1 i u s preserved 

 phistically in the Vaginatenkalk of Oeland and by means of thin sections 

 proved that the gemmation of the first thecae and the formation of the four 

 brandies of the rhabdosome take place in exactl}' the same manner as in Tetra- 

 graptus I text. ])A7(^ \. The sicula is embedded at the distal end of the rhab- 

 dosome, the four 1:»ranehes growing backward from the initial point toAvard the 

 point of fixation, as in Te t r a g r a p t u s (]) i g s 1) }' i) s i m i 1 i s . Between 

 this species and W a n gu s tif ol i us there exists the closest agreement, 

 hoth in internal and external structure. In Phyllograptus, however, the four 

 branches have coalesced, so that the four coenosarcal canals form a quadi'i- 

 partite internal tube, and the four independent periderm Avails of Tetra- 

 graptus si mi lis ha\"e united into "a single, cruciform, four winged, 

 longitudinal septum " | section, iig.94(:/]. The longtitudinal partitions of the 

 central tube consist, hence, of a single layer. 



The matei'ial from tlie Deep kill consists largely of flattened s])ecimens in 

 slate. Among them tliei'e occur finely macerated examples in a Ja\e]' of the 

 (piariT beds Ipl.l."), fig.33], which show the interthecal walls in excellent 

 preservation. In a hiyer of graptolite bed 2 the specimens of P . i 1 i c i f o 1 i u s 

 were laigely preserved plastically by having become the centers of j)yrite 

 nodules. Sections through these have permitted us to investigate the internal 

 structure of that species. 



Hall suggested that the fronds or rhabdosomes of Phyllograptus may 

 have been attached in groups to some other sup]3ort, a suggestion Avhich 

 was based on their arrangement on a slab figured on plate 15 (figure 10) of 

 his work. This arrangement is, however, accidental, as the writer had 

 occasion to satisfy himself, and, as is indicated by the irregular distribution 

 of the antisicular extremities of the rhabdosomes. The mode of occurrence 

 of Phyllograptus would rather suggest that the rhabdosomes as found now 

 constituted the entire colony, only the organ of suspension having been lost 

 or failed of preservation. 



