30 



NORTH AMERICAN INDEX FOSSILS. 



hydrothecae to an inch. Broad axis or midrib, often crenulate or 

 serrate. 



Lower graptolite shale (Upper Beekmantown) Canada and Hud- 

 son Valley. 



6. P. ilicifolius Hall. (Fig. 45.) 



Lower Ordovicic. 



Fig. 44. Phyllograp- 

 tus typus (after Roemer), 

 enlarged. 



Fig. 45. Phyllograptus ilicifolius. a, natural size; 

 b, enlarged ; and c, restored crosssection (after Roe- 

 mer). 



Differs from the preceding in its thicker substance, proportion- 

 ally shorter and broader form, more closely arranged hydrothecae 

 (29-32 to an inch) and strongly mucronate or spinous lower lip of 

 hydrothecae. 



Shales of Middle Beekmantown age, Canada and New York. 



17. P. angustifolius Hall. (Fig. 46.) 



Lower Ordovicic. 

 Narrow elongate form, generally a little 

 broader at the base. Thecae strongly mu- 

 cronate on lower sides. Lower shales (Beek- 

 mantown) of Point Levis and Hudson Valley. 



18. P. anna Hall. Lower Ordovicic. 

 More nearly rounded, often truncated, nu- 

 merous thecae. Higher Beekmantown and 

 Lower Chazy horizon. 



^ 



Fig. 46. Phyllograptus 

 angustifolius, enlarged 

 (after Roemer). 



X. DlDYMOGRAPTUS McCoy. 



Rhabdosome consisting of two symmetrical 

 branches diverging from the basal cell (sicula) at angles from io° 

 to 180 or over, with oblique hydrothecae opening inwards. Or- 

 dovicic. 



