GRAPTOLITES OF NEW YORK, PART 2 io7 



NOTES ON PHYLOGEN\ 

 As the headline implies, it is not intended to give in this place a full 

 account of the phylogenetic relations of the forms here described, but only 

 to bring- together a number of notes which have suggested themselves dur- 

 ing the prosecution of this work and which bear on the probable phylogeny 

 of these forms. 



a On the Phylogeny of the Leptograptidae and Dicranograptidae 

 The genera Leptograptus and Didymograptus are so similar in their 

 general appearance that the former was not recognized as a separate 

 genus until Lapworth with characteristic keenness discerned the differences 

 in the morphologic characters of their thecae, Didymograptus having the 

 primitive straight tubular theca while that of Leptograptus has a gentle 

 sigmoid curvature and a slightly introverted aperture opening in a shallow 

 excavation of the next theca. Later the same author has united all genera 

 grouping themselves around Leptograptus by this form of the thecae in the 

 family Leptograptidae. 



Elles and Wood have pointed out that there is a tendency to the 

 development of this character already observable in some species of Didymo- 

 graptus, as D. acutidens. The same authors show that also another 

 difference, consisting in the presence of two crossing canals in the Lepto- 

 grapti against one in the Didymograpti loses its character as absolute 

 differential character by the presence of two crossing canals in D. gib- 

 berulus. By the existence of such connecting links the derivation of 

 the Leptograptidae from Didymograptus is distinctly demonstrated. 



The compound forms of the Leptograptidae have here been arranged 

 in the genera : Sigmagraptus, Nemagraptus, Syndyograptus, Amphigraptus 

 and Pleurograptus. We have pointed out in Memoir 7 the phylogenetic 

 relations obviously existing between Sigmagraptus and Nemagraptus. 

 The genera Syndyograptus and Amphigraptus possess a peculiar character 

 in common in the paired disposition of the secondary branches, a feature 

 which distinguishes them from all other multiramous forms. Of the two, 

 Syndyograptus is the more primitive in appearance, by the more regular 



