GRAPTOLITES Ol M.W YORK, PART 2 Jig 



group (if D. furcatus. We have more fully discussed these transitional 

 characters in the chapter on the phylogeny of the genera, page 109. 



The fact, mentioned in the description oi this species, of the alternate 

 arrangement of the thecae on the inside and outside of the branches, indi- 

 cates, together with the convergence of the branches, that the latter grew 

 in a slender spiral, comparable to that of I), furcatus. The not infre- 

 quent occurrence of specimens, like those reproduced in plate 19, figures 5 

 and 6, where the branches cross already close to the sicular extremity, while 

 evidently due to oblique compression, can only be understood if it is 

 assumed that the branches lay from the beginning in different planes. 



DICRANOGRAPTUS Hall 



Hall separated Dicranograptus first [1865, p. 112] as a subgeneric group 

 from Climacograptus pointing to the similarity of the thecae in Gr a p to- 

 lit h u s ramosus, the type of the new group, and Climacograptus 

 bi co rn is. In the 20th Museum Report, in the Synopsis of the Genera 

 of Graptolitidae, Dicranograptus is given full generic rank but expanded 

 so as to comprise also the present Dicellograptus. The latter having again 

 been separated by Hopkinson, the genus Dicranograptus now embraces all 

 forms which have the characteristic Dicranograptid thecae (with more or 

 less convex ventral walls, apertural introversion and introtortion) and whose 

 rhabdosome consists of a proximal biserial portion and two distal uniserial 

 stipes. 



Like Dicellograptus it appears abruptly and attains a sudden domi- 

 nance in the Trenton shales, w r here by the number of individuals and still 

 more by the size of its robust rhabdosomes, it is one of the most promi- 

 nent genera, and disappears again as rapidly in the succeeding formation. 



The form of the complete rhabdosome — or eventually synrhabdosome 

 — and its mode of suspension and fixation, as well as the probable origin 

 of the biserial section and the question of the presence of an axis (virgula) 

 have been discussed in the introductory chapters. Also in regard to the 

 phylogenetic relations of the genus we refer the reader to page 109. 



