28 



NORTH AMERICAN INDEX FOSSILS. 



VI. Cgenograptus Hall. 

 Bilaterally symmetric rhabdosome, the main stem (of tubular 

 hydrothecae) bent into an S with simple unicellular branches of 

 similar structure given off from the convex 

 side. The sicula appears at the center of 

 the principal stem. Ordovicic. 



9. C. gracilis Hall. (Fig. 40.) 



Middle Ordovicic. 

 Slender stem, branches with fine serra- 

 tions from crowding of the hydrothecae, fig. 40. Cxnograptus 

 Normanskill shales, Hudson Valley, etc. gracilis. 



VII. Dichograptus Salter. (Including Loganograptus 

 and Gon iograptus. ) 

 Rhabdosome of eight (typical) or more (Loganograptus and 

 Goniograptus) large and narrow branches, united by a common 



stem in center which with the lobes 

 of the branches lies within a central 

 membranaceous disc. Branches 

 with single row of hydrothecae, be- 

 coming prominent some distance 

 from center. When branches occur 

 separated, they can only be deter- 

 mined by the form of their hydro- 

 thecae. Ordovicic. 





Fig. 41. a (upper), Loganograptus 

 logani ; b (middle \eh),Tetragraptus 

 quadribrachialus ; c, Dichograptus 

 octobrachiatus ; d (lower), Gonio- 

 graptus thureaui (all enlarged). 



10. D. octobrachiatus Hall. (Fig. 

 41.) Lowest Ordovicic. 



Eight, large and coarse branches often a foot long — central 

 disk eight-angled. 



Lower shales, Point Levis and Hudson River Valley, Beekman- 

 town horizon. Also European. 



11. D. (Loganograptus) logani Hall. (Fig. 41, a.) Lowest 



Ordovicic. 



Numerous (thirteen to twenty-five, normally sixteen) slender 

 branches up to nine inches long, symmetrically branching at base. 

 Hydrothecae become prominent only at some distance from base. 

 Central disk many-angled. 



Lower graptolite shales (Beekmantown), Point Levis and Hud- 

 son River Valley. Also European. 



