GRAPTOLITES OF NEW YORK. PART 1 681 



desciibes it from the Phyllograptiis shales of Skattimgbyn in Dalarne, 

 Sweden. 



Remarlis. This is ])}' all means the most delicate and most attenuated 

 species of Didymograptus which we have noticed in the New York fauna or 

 found lecoided from other regions. The tendency toward the development of 

 long slender thecae and narro\v flaccid branches, which appears first in 

 G o n i o g r a p t u s p e r f 1 e x i 1 i s, has here apparently reached its most typi- 

 cal expression and also its termination. The distal portions of the rhabdosomes 

 are frequently so fine that they are barely noticeable with the naked eye, and 

 the lithographer did not produce these in their actual thinness for fear the 

 lines could not be printed. The enlargement gives a better idea of this 

 slenderest of all graptolites. 



It has been stated in the preliminary paper on the Deep kill fauna, cited 

 above, that the earlier thecae of this form are considerably shorter than the 

 later ones (text p.583), and the phylogenetic significance of this feature has 

 been pointed out. It Avas considered as indicating derivation of the species of 

 Didymograptus with long, narrow thecae from forms with shorter and broader 

 thecae. The same difference in the thecae is shown on figui-e 11 of Tullberg's 

 drawings. In the Deep kill material tlie earlier thecae have a length of but 

 . 8 mm, Avhile the later ones attain 2 mm in length. These are six to eight 

 times as long as wide and appear tubular for two thirds of their length. The 

 more proximal thecae, however, agree \vell with the enlargement given by 

 Tullberg. 



While our specimens, in the slenderness of the mature branches, the long 

 tubular character of the thecae, their small overlap and inclination, seem to 

 represent an extreme development of the characters described by Tornquist, 

 or I'ather go beyond the t3'pical Swedish form in development, the form 

 referred to this species in England has more rapidly widening thecae and a 

 correspondingly greater angle of inclination than the Swedish type and 

 varies hence in the opposite direction to that of the Deep kill form. 



