152 NORTH AMERICAN INDEX FOSSILS. 



regular intervals; apertures in two ranges on sides of stipe; re- 

 verse of branches finely striated. 

 Chester of Illinois, etc. 



LXV. Arthrostylus Ulrich. 



Bushy, dichotomously divided branches which consist of nu- 

 merous, exceedingly slender, equal subquadrate segments, united 

 by terminal articulations ; one face longitudinally striated, the 

 other three (sometimes less) with a linear series of apertures generally 

 between longitudinal ridges. Ord. 



115. A. obliquus Ulrich. (Fig. 205, i.) Ordovicic. 

 Segments needle-shaped, about 4 mm. long ; subquadrangular 



in section (0.2 x 0.15 mm.) slightly expanding to extremity; 

 apertures oblique, lower border prominent; no ridges between 

 ranges. 



Stones River of Minnesota. 



LXV I. Helopora Hall. 



Differs from preceding in its larger segments, which have zocecial 

 apertures on all sides. Ord.-Sil. 



116. H. spiniformis (Ulrich). (Fig. 205,7'.) Ordovicic. 

 Segments 5 to 10 mm. in length, obtusely pointed at the ends; 



apertures oblique on all sides, in from 

 8 to 16 longitudinal ranges, and sep- 

 arated by slightly elevated lines. 



Stones River of Tennessee and Illi- 

 nois. 



117. H. fragilis Hall. (Fig. 207.) 



Siluric. 

 Swollen at one end, apertures oval , .,. 



Pig. 207. Helopora fragths i 



or subangular, having a spiral direc- natural size and enlargecL 

 tion around the stipe ; length 5-6 mm. 



Clinton and Niagara of New York and Ontario. 



LXVII. Arthroclema Billings. 



Segments celluliferous on all sides, articulated laterally and termi- 

 nally in a pinnate manner ; apertures oval in series between longi 

 tudinal ridges. Ord. 



