32 



NORTH AMERICAN INDEX FOSSILS. 



22. C. bicornis. (Fig. 50, 51, a.) Middle Ordovicic. 

 Gradually widening upwards, hydrothecae approaching rectangu- 

 lar outline, base with two diverging mucronate points. 



Normanskill shales (Trenton) of Hudson 

 Valley, and equivalent beds of the Cincinnati 

 region. 



23. C. typicus Hall. (Fig. 51, b.) 



Upper Ordovicic. 



Differs from preceding in narrow almost 

 horizontal incisions between, and small mucro- 

 nate point at base of hydrothecae. 



Utica Slate of New York and Cincinnati re- 

 gion. 



XII. DlCRANOGRAPTUS Hall. 



Lower portion of hydrosome biserial, upper 

 portion dividing into two uniserial branches. 



Form Y-shaped. Hydrothecae as in Cli- 

 macograpttts. Ordovicic. 



Fig. 50. Clintacog" 



rapt us bicornis (slightly 

 enlarged). 



24. D. ramosus Hall. (Fig. 52.) 



Undivided portion Ion 



Middle Ordovicic. 



incisions between 

 hydrothecae deep, the latter with mucronate 

 points. 



Normanskill shales (Trenton), Hudson Val- 

 ley. Europe. 



Fig. 51. a (left), 

 Climacograptus bicornis ; 

 b, C. typicus (enlarged). 



XIII. Dicellograptus Hopkins. 

 Like the preceding, but divided to the base 

 V shaped. Ordovicic. 



25. D. complanatus Lapworth. (Fig. 53, a,b.) 



Middle Ordovicic. 



Diverging at angle of 30-50 degrees. Hy- 

 drothecae narrow and very oblique ; non- 

 mucronate. 



Normanskill shales (Trenton), Hudson Val- 

 ley. 



26. D. divaricatus Hall. (Fig. 53, c.) 



K & JU J Fig. 52. Dicranog- 



Middle Ordovicic raptm ramoms (sligMly 



Diverging at angle of over 90 , up to I 30 ; enlarged). 



