GRAPTOLITES OF NEW YORK, PART 1 



mi 



evidence of the derivation of T. taraxacum from a form with the 

 thecal characters ofB.lapworthi.^ 



B r y o g r a p t u s 1 a p w o r t h i can be considered as having descended 

 through unknown intermediate forms from Staurograptus dicho- 

 t o m u s var. a p e r t u s with some degree of certainty, springing from the 

 fact that the character and arrangement of the thecae in the two species fairly 

 agree (number of thecae 11-13 in 10 mm in S. dichotomus var. 

 apertus; angle of inclination 25°; overlap ^). 



(8) A series with strong diagnostic characters is that leading from 

 Goniograptns perflexilis through a peculiar iiexuous, four 

 branched form, Tetragraptus lent us, to Didymograptus 

 f i 1 i f o r m i s and D. gracilis. 





Character of thecae etc. 



Number 



of thecae in 



]0 mm 



Angle of 

 inclination 



Overlap 



Width 

 •of branches 



Goniograptns 

 perflexilis 



Thecae extremely 

 long, narrow, 

 very little widen- 

 ing, curved. 

 Branches of first 

 order originating 

 near middle of 

 sicula 



6-7 



5° 



i 



A 



Tetragraptus 

 1 e n t u s 



6-10 



5° 



1 



.37 



Didymograptus 

 filiformis TuUb. 



8-10 



10°-15° 



i 



.25 



The specific names of all four species express the thin, iiexuous 

 character of the branches, which is due to the extreme length (3 mm) and 

 narrowness of the thecae, as well as to their small overlap (one fourth) and 

 loose arrangement (6-10 in the space of 10 mm). 



A peculiar character which these forms have in common with the 

 Coenograptus series, also to be derived from Goniograptus per- 

 flexilis, is the origin of the branches about midway of the sicula. 



^ The change in the character of the thecae of T. taraxacum has been discussed 



by the writer in another connection [1902, p.589]. 



