200 



ECHINODERMATA— PELMATOZOA 



PHYLUM XV 



and incorporated into the tegmeri. Anus either eccentric, or at the end of a 

 thick, usually short tube. Arms branching once to three times, uniserial in 

 the lower parts, but becoming biserial distally. Column elliptical and twisted ; 

 the axes of the upper and lower surfaces of the individual segments being at 

 a slight angle with one another; central canal very minute; strong cirri 

 toward distal end. Rare in Devonian ; abundant in Lower Carbonif erous of 

 England, Belgium and North America (Hamilton to St. Louis Groups). 



Brahmacrinus Sollas. Like Platycrinus, but radials relatively smaller, and 

 lower brachials larger, and incorporated in dorsal cup to the height of the 

 second distichals. Lower Carboniferous ; England. 



Eudadocrinus M. and W. Calyx like Platycrinus, but with rays produced 

 into one or two main rami, roofed with rigid covering plates, forming tubulär 

 extensions of the calyx, giving off at alternate sides biserial, pinnuliferous 

 arms. An extreme development of the family type, but not the latest in 

 time. Lower Carboniferous (Kinderhook to Keokuk) ; North America. 



Family 10. Hexacrinidae Wachsmuth and Springer. 



Monocydic. Radials in contaä except at the posterior side, where they are 

 separated hy an anal plate. Basais three, or two, forming a hexagon. Strudure 

 otherwise as in the Platycrinidae. Devonian to Lower Carboniferous. 



§ 1. Basais three. 



Hexacrinus Austin (Fig. 302). Costals two, united by syzygy, or one. 

 Rays usually in two main trunks, bearing uniserial pinnulate arms on one or 



Fig. 302. 



Hexacrinus elongatus (Goldf.). Devonian ; Pehn, Eifel. a, Calyx seen from one side ; /;, Aspect of summit ; 

 c, Analysis of calyx and arms ; d, e, Column of //, spinosus (Müll.) (after L. Schnitze). 



both sides at intervals. Tegmen as in Platycrinus. Column round, with small 

 axial opening. Devonian. Represented by numerous species in England, 

 Belgium, and the Eifel, but by only a few in North America. 



Arthracantha Williams (Histricrinus Binde). Calyx as in Hexacrinus, but 

 the plates covered with movable spines borne on tubercles. Arms dichotomous, 

 biserial. Stern circular. Devonian : New York and Canada. 



