I IO 



NEW YORK STATE MUSEUM 



few of the proximal thecae and for some other minor differences. But the 

 vast majority of the specimens differ distinctly from D . sextans by hav- 

 ing the basal parts of the branches subparallel and closely approaching 

 each other. Yet a faint tendency to this approach is already noticeable in 

 D. sextans. Further, D. smith i is distinguished from D. sextans 

 in not having the thecae on the inside of the branches, but alternating on 

 the inside and outside [see fig. 48]. This fact and that of the convergence 

 of the branches indicate that the branches grew in a very slender spiral, 

 that leads to the more contracted spiral of forms like Dicranograptus 

 f u r c a t u s. Again a slight tendency to a torsion of the branches has been 

 observed in a variety of D . sextans, here described as D . sextans 

 var. tortus. We can, therefore, infer that slight variations already 

 noticeable in the typical sextans and its varieties of the New York 

 Normanskill shale took in the Alabama form a sudden spurt and led to a 

 new species belonging to the more advanced genus Dicranograptus. 



To show the close relationship of the three species, we compare their 

 principal characters which pertain to their general appearance in the 

 following table. 







'— 



1 ,—s 





tr. 



. 













X w 





H 



X 

 u 



■=. 5 









w 



tn 2 





?5 



h ° 







FORM OF 

 RHABDOSOME 





 ■z 



O 

 Pi 

 W 



X t 



< < 



a: j 



c 

 X 



< 



CHARACTER OF 

 THECAE 







> 





W 





g O 









P 





** 





Z 





Dicellograptus 



Very slender spiral. 





6o Q 





.8 



II-I3 



Short, convex ven- 



sextans 



Slight proximal 

 concavity 













tral walls, aper- 

 tural part intro- 

 torted 



Dicellograptus 



Slender spiral 





6o Q 





.6 1 



12-14 



Short, convex ven- 



smithi 



brandies, prox- 

 imally parallel 













tral walls, aper- 

 tural part intro- 

 torted 



Dicranograptus 



Branches forming 



40 



-90 



•9" 



[.1 



12-14 



Short, convex \ en- 



farcatus 



short spiral, bases 

 coalescent 













tral walls, aper- 

 tural part intro- 

















torted 



'Less llattened out. Preserved in limestone. 



