4 8 



NORTH AMERICAN INDEX FOSSILS. 





ioo. A. laxus Hall. Mississippian. 



Very loosely coiled, axis formed of thickened edge of frond, one 

 volution in 25 mm. or more ; angle of fenestrated portion acute. 



Chester of Illinois and Kentucky. 



10 1. A. sublaxus. (Fig. 205, b.) Mississippian. 

 About half the size of the preceding. 



Chester of Illinois. 



102. A. terebriformis Ulrich. Mississippian. 

 Differs from A. communis in being more slender, with from 3 to 



4 volutions in 20 mm. In coiling resembles A. laxus. Angle of 

 divergence 60 to 65 degrees. 



Chester of Illinois and Kentucky. 



LVIII. Polypora McCoy. 

 Differs from Fenestella in having from two to eight rows of 

 zocecia on a branch, and the median keel reduced to a row of 

 strong nodes or tubercles, or absent altogether. Sil.-Carb. 



Fig. 204. Polypoi-a incepta with non-celluliferous and celluliferous faces enlarged. 



103. P. incepta Hall. (Fig. 204.) Siluric. 



Branches regularly dividing with flexuous striae on the non- 

 celluliferous side, and several strong ridges separating the three or 



