200 



ECHINODERMATA— PELMATOZOA 



PHYLUM IV 



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 uj^per and lower surfaces of the individual segments being at 

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

 toward distal end. Eare in Devonian ; abundant in Lower Carboniferous of 

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



Brahmacrinus Sollas. Like Platycriims, 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 Flatycrimis, but with rays produced 

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

 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 Waclismuth and Springer. 



Monocyclic. Eadials in contact excejyt at the posterior side, where they are 

 separated by an anal plate. Basals three, or two, forming a hexagon. Structure 

 otherwise as in the Platycrinidae. Devonian to Lower Carboniferous. 



§ L Basals three. 



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

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



Fig. 302. 



Hexacrinus elongatus (Goldf.). DRVouian ; Pelm, Eifel. a, Calyx seen from one side ; h, Aspect of summit ; 

 c, Analysis of calyx and arms ; d, e, Column of //. sxtinosus (Miill.) (after L. Schultze). 



both sides at intervals. Tegmen as in Plafycrimis. 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 Hinde). Calyx as in Hexacrinus, but 

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

 biserial. Stem circular. Devonian ; New York and Canada. 



