520 NORTH AMERICAN INDEX FOSSILS. 



66. P. hemisphericus Meek and Worthen. (Figs. 1842, 1806, a.) 



Mississippic. 



Sutures of calyx depressed but not grooved. Plates covered 

 with rounded nodes. 



Keokuk of Indiana and Iowa. 



6y. P. huntsvillae Troost MS. (Figs. 1843, 1806, b, c.) Mississippic. 



Sutures of the lower brachials with one another waving or zig- 

 zag. Arms frequently inflated in the middle portion. Stem joints 

 bearing conspicuous nodes. Interbasal sutures raised into ridges. 



St. Louis of Alabama and Illinois. 



68. P. sarae Hall. Mississippic. 



General shape of calyx much as in P. huntsvillce, but more 

 broadly truncate and slightly concave basally, with a rim project- 

 ing over the stem. Arms, six in each ray, tapering. Pinnules 

 very long and close. Plates smooth or obscurely ridged. 



St. Louis of Tennessee, Illinois and Missouri ; Warsaw of 

 Illinois. 



XXVIII. Arthracantha Williams. {Histricrinus Hinde.) 



B 3, equal, forming a cup hexagonal in outline and supporting 



6 plates (5 R and a large anal plate between the two posterior R). 



R large, excavated above for the two costals and with slightly 



truncated limbs for the reception of the IR. 



Plates covered with numerous tubercles 



each with a small pit for the reception of a 



movable spine. Arms branching, biserial. 



Column round. Devonic. 



Fig. 1844. Arthracantha ^g^ A. punctobracMata Williams. (Fig. 



punctobrachiata, side view. n \ t^ • 



(After Whiteaves.) '844.) DevoniC. 



Interbrachials 3 to each regular inter- 

 ladius and followed by many rows of small interambulacral pieces 

 which meet on the summit, as there are apparently no orals. 

 Hamilton of Ontario. 



XXIX. DicHOCRiNUS Miinster. 

 Calyx oblong to almost cylindrical. Dorsal cup consisting almost 

 wholly of B, R, and a large anal. Plates delicate. B 2, large, the 



