GRAPTOLITES OF NEW YORK, PART 2 67 



vitality but of our failure to distinguish the different groups of possibly 

 different internal structure but like external appearance. The same could 

 be said of Desmograptus. 



In contrast to these long-lived genera stands a group of Niagaran 

 Dendroid genera (Cyclograptus, Inocaulis, Acanthograptus, Cactograptus, 

 Palaeodictyota) of very short range. The whole range of these groups is 

 probably not known, especially may they be older than is indicated by their 

 observed range. This is suggested by the appearance of a form (D e n- 

 drograptus ? succulent us) in the third Deepkill zone that has 

 many characters indicating its ancestral relations to Inocaulis, Acantho- 

 graptus and Palaeodictyota. 



The genera of the Axonolipa are nearly all quite short-lived. The 

 synoptic table clearly brings out the fact that they appear in two different 

 successive outbursts. The first in early Beekmantown time (Zone of Teta- 

 graptus) with the genera Temnograptus, Goniograptus, Loganograptus, 

 Dichograptus, Tetragraptus, Phyllograptus and Didymograptus. Only 

 the last-named of these genera extends into Mesochamplainic or Mohawkian 

 time. But there another group of genera appears mainly forming the fami- 

 lies Leptograptidae and Dicranograptidae, viz, Azygograptus, Leptograp- 

 tus, Syndyograptus, Pleurograptus, Amphigraptus, Nemagraptus, Dicello- 

 graptus and Dicranograptus. Nearly all of these (except Azygograptus) 

 show their progress beyond the Dichograptidae of the Paleochamplainic 

 by the sigmoid curvature of their thecae. A genus (Sigmagraptus) of the 

 same group standing in front of them in the preceding column of the table 

 suggests already by its position the ancestral relations demonstrated by its 

 form. Few of these genera lived in Neochamplainic time and none passed 

 beyond the end of the Champlainic. 



Proceeding down the table, we come again to a pronounced projection 

 of the genera into an earlier zone in the Axonophora, where the genera 

 Diplograptus, Glossograptus, Trigonograptus, Climacograptus, Crypto- 

 graptus and Retiograptus appear together, ushering in the new climacteric 

 period of graptolites (Diplograptidae) in the Trenton. The beginning of 



