GRAPTOLITES OF NEW YORK, PART 1 695 



Diplograptus dentatus). They will be described iu addition to 

 this species. 



In northern Europe this species has been well known for a long time. In 

 England it occurs in the Upper Tetragraptus beds of the Middle Skiddaw 

 slates, hence in the equivalent of the horizon to which it probably belongs in 

 the Deep kill section. It is also reported from the same horizon in the shale 

 of the St David's district iu Wales (P h y 1 1 o g r a p t n s s t e 1 1 u Hopkinson). 

 In Sweden it characterizes a horizon of the lower 

 Graptolite shale of Scania and Westrogothia [Torn- 

 cpiist, loc. cit.1. Holm had isolated material from the 

 Vagiuatenkalk of Oeland. Freeh mentions it as 

 having been collected neai- Chi-istiania by F. Roemer. 

 It is also characteristic of the lower graptolite 

 shales of Victoria [McCoy, Etheridge jr] and New- 

 Zealand, Australia [Jide Freeh]. It has not been 

 recorded from the Bohemian basin. 



Remarhs. This species of striking appearance 



Fig. 89 Didymojrraptus cad- 



and taxonomic importance has been elaborately Sbvel4*'4*w;" ii^ep km xir™"' 

 described by Moberg, Elles, Tornquist and Lap- 

 worth, Elles and Wood. Our material is not sufficient to verify all the observa- 

 tions of these authors, specially in regard to the central parts of the rhabdosome, 

 much less to add to their descriptions. We learn from these investigators 

 that the first theca originates very near the apex of the sicula and follows the 

 latter to a point near the ai)erture of the sicula, whei'e it bends away from 

 the latter. Also the second theca is nearly as long as the first, so that the 

 connecting canal must lie veiy high up. 



Moberg proposed to make this species the genotype of a new genus, 

 Isograptus, on the ground that in this form the branches arise bilaterally sym- 

 metric from the sicula, and each branch is not itself bilaterally symmetric, 

 while in Didymograptus both branches arise at somewhat different levels at 

 the sicula, and each branch is itself bilaterally symmetric. Holm has however 

 Bhown the relations of branches and sicula to be the same in Didymograptus 



