26 



NORTH AMERICAN INDEX FOSSILS. 



Branches very slender, bifurcating and slightly diverging, irreg- 

 ularly striated or indented. Dissepiments slender and distant. 

 Occurs with the preceding. 



4. D. hamiltoniae. Hall. (Fig. 35.) Mid-Devonic. 



Fig. 35. Dictyonema hamiltonice with enlarge- FlG. 36. Desmograptus cancel- 

 ment. latus (Ruedemann, N. Y. State 



Mus. Mem. ). 



Branches slender, bifurcating, somewhat flexuous and uniting as 

 in Desmograptus. Dissepiments scattered. (This may belong to 

 the next genus.) 



In the Hamilton shales of New York and in similar beds in 

 Michigan. 



II. Desmograptus Hopkins. 



Differs from Dictyonema in the flexuous character of the 

 branches, which unite at intervals when they come in contact. 

 Dissepiments chiefly in lower part of frond. Ord.-Dev. 



5. D. cancellatus Hopkins. (Fig. 36.) Lower Ordovicic. 

 Branches 12 to 14 in 10 mm. forming long narrow meshes, 



twice as long as wide. Thecal apertures circular. 



In shales of the age of the Lower Chazy in New York and 

 Canada (?). 



III. Dendrograptus Hall. 



Rhabdosome with a strong main stem supporting a broad, 

 spreading, shrub-like, variously ramifying frond. Hydrothecae 

 commonly obscure, or in the form of pits, sometimes well marked. 

 Ordovicic. 



6. D. flexuosus Hall. (Fig. 37.) Lower Ordovicic. 

 An inch or more in length, branches broadly and rather regu- 

 larly diverging. 



