LOWER SILURIC SHALES OF THE MOHAWK VALLEY 



79 



Fig. 1 8 



The main differences from the typical n i c h o 1 s o n i are the 

 closer arrangement or smaller size of the thecae, which number 14 

 in 10 mm, and the greatly varying length of the biserial portion 

 (4-11 mm). The form of the thecae does Fig _ I? 

 not seem to differ from that of the larger ^ 

 types. The biserial portion has been seen f\ 

 in only one specimen and in this the axillary 

 angle is smaller than in nicholsoni 

 t y p u s , namely about 10. 



The width of the stipe is not materially 

 different from that of nicholsoni. 



The most remarkable feature of the spec- 

 imen retaining the biserial portion is the con- 

 tinuation of the axes of the biserial branches 

 in antisicular direction. These axes have to 

 our knowledge not been observed in either 

 Dicellograptus or Dicranograptus outside of 

 the theciferous portion. The free portions 

 are too long and delicate to have been theci- 

 ferous and to have secondarily become sepa- 

 rated from the thecae. The rhabdosome of 

 Dicranograptus was hence suspended from 

 two axes which, as the parallel and slightly 

 convergent final direction of the axes sug- 

 gests, proceeded from a common initial point. 



The specimen exhibits also a large " web " 

 between the branches. 



Dicellograptus nicholsoni 

 possesses in the slate belt of New York a 

 much larger vertical range than we had in- 

 ferred in Memoir n, page 318. We had 

 cited it there from the " Trenton shales " 

 (Normanskill shale) in several varieties and 



in typical development from the Utica of ing axes or nemacauli, 

 South Trenton, the Mohawk valley and the x 5. Fig. 18. Long 

 shales at Saratoga lake, which were also biserial portion of sped- 



' men showing form of 



supposed to be of Utica age. The shales thecae, x 5. 



which carry D. nicholsoni in the Mo- Canajoharie shale, 

 hawk valley are in the present paper referred Morphy creek, N. Y. 

 to the Trenton as Canajoharie shale and those at Saratoga lake are 

 also correlated with the Trenton as Snake Hill beds. The Normans- 



Fig. 17, 1 8. 

 Dicranograptus 



nicholsoni 

 Hopkinson var/ 

 p a r v u 1 u s nov. 



Fig. 17. Specimen show- 



