GRAPTULITES OF NEW YORK. PART I 



661 



series of originally multiramous forms. The genus is hence like Tetragraptus 

 pol^^phyletic and expresses a stage reached nearly simuHaneously along many 

 lines of evolution. It, accordingly, falls naturally into groups ^v■hich probably 

 unite species of actual genetic relationship. These groups have, through 

 the larger and more varied association of British Didymograptidae, recently 

 been determined by the monographers of the fauna of that country. They 

 have, again, by common characteristics of general habit, been arranged into 

 series. These are based largely on the divergence of the branches, which 

 compasses the entire circumference of a circle [see diagram, p.485j, probably 

 in consequence of the endeavors of the branches to assume progressively a 

 more and more erect position from the point of suspension (the sicula). 

 We have adopted here for the arrangement of our species of Didymograptus 

 the grouping proposed by Elles and Wood. 



Group 1 Type : D. extensus 



D. extensus 



D. n i t i d u s 



D. p a t u 1 u s 



D. s i m i 1 i s 



D. gracilis 

 Group 2 Not represented 



Groups Type: D. affinis 

 D. e 11 e s i 

 D. acutidens 

 D. cuspidatus 

 D. nicliolsoni var. planus 

 D. f i 1 i f r m i s 

 D. tornquisti 

 D. spinosus 



Group 4 Not represented 



Group 5 Type : D. murchisoni 



a Subgroup of D. b i f i d u s 



D. bif idus 

 i Subgroup of D. in dent us 



L). nanus 



^Horizontal series 



Declined series 



> Dependent series 



J 



