GBAPTOLITES OP NEW YORK, PART 1 



i>i)i 



T. fruticosus, in which, by the process of acceleration in development, 

 the flexure and widening of the branches have been transfeiTed to their 

 proximal portions. 



(2) Didymograptus similis agrees in the characiters of its thecae so 

 closely with Did. extensus that it can safely be held to be a descendant 

 of that species which has fallen belo\v the size which the series attained at the 

 time of its acmic development in the Tetragraptus horizon. Did. similis 

 is found in the next two horizons, that with Did. b i f i d u s and that with 

 Diplograptus dentatus. It possesses inconspicuous sicula, the broad 

 and short thecae of Did. e x t e n s ii s , the thecae number in both 

 9-10 within the space of 10mm, and the angle of inclination and overlap 

 of the thecae do not differ materially ; the overlap of the thecae is a little 

 greater (two thirds to three fourths against one half to two thirds in Did. 

 extensus), indicating a slight condensation of the thecae, while at 

 the same time the total length of the branches has become greatly lessened. 



(3) Didymograptus nitidus holds a position between D . 

 p a t u 1 u s and D. extensus, both of which it approaches in some 

 of its many slight varieties. With D . extensus it has in common the 

 size of the sicula, inclination and overlap of the thecae, but difiFers in the 

 considerably closer arrangement of the thecae. There is no Tetragraptus 

 known in the Lower Siluric to which D . nitidus could be readily 

 referred ; and we incline to the opinion that it is a derivative from another 

 species of Didymograptus, possibly D. extensus. 



(4) Bry ograp tu s 1 ap wort hi, Didymograptus nichol- 

 s o n i and D . e 1 1 e s i . 



Bryograptus 



lapworthi 

 Didymograptus 



nichol son i 



Did. ellesi 



Characters of thecae etc. 



Slender forms ; thecae 

 with straight or slight- 

 ly concave outer walls ; 

 apertures appear near- 

 ly straight 



Number of thecae 

 in 10 mm 



10 

 10-11 



10-12 



Inclination of 

 thecae 



15-20° 

 20° 



8-10° 



Overlap 



i-i 



