2'78 PllOCEEDINGS OF THE ACADEMY OF [1881. 



iiuh<r mis CENTROCRINUS Waclism. and Spr. 



This genus sliould not be confounded with Centrocrinus Austin,' 

 wliich in 1843, was proposed to distinguish a certain form of 

 Platijcrinus, but based altogether upon conjecture and incorrect 

 observation. 



The generic form now suggested embraces at present only two 

 species, Lyon's Actinocrinus pentaspinus and his A. muUicoryius, 

 of which we propose the former as the type. The two differ from 

 Actinocrinus essentially in the construction of the anal area, 

 having no plate in line with the first radials. The nearest allied 

 genus, and under wliich we place it subgenerically, is Macrostylo- 

 crinus, but this has three anal plates in the second series, in place 

 of two, while in Centrocrinus the posterior side is not in any way 

 distinct from the other four. The spiniferous form of some of 

 the body plates has suggested the name. 



Generic Diagnosis. — Calyx subcylindrical ; s^anmetry perfect!}' 

 pentahedral; the principal plates produced into spines ; the calyx 

 in its lower portions almost resembling Platycrinus, owing to the 

 form and size of basals and first radials. 



Basal disk large, pentagonal, composed of three unequal pieces, 

 two of them pentangular, and twice the size of the third, whicli 

 is quadrangular. 



Primary radials 3X5; the first very large and spiniferous ; 

 second radials much smaller than the first, short and quadrangular 

 or nearly so ; third radials pentangular, sometimes triangular, as 

 short as — and occasionally narrower — than the second. Secondary 

 radials 1X10, bent abruptly outwai'd and supporting the arms. 

 There are, so far as known, two arms to each ray. 



Interradials four to five ; the first large and frequentl}' spinifer- 

 ous, deeply inserted between the first radials, in which the upper 

 lateral margins are exceptionally long ; second series composed 

 of two plates, much smaller than the first ; third series consisting 

 of one or two plates, located between the arm bases. Anal area 

 not distinct. Construction of vault and arms unknown. 



Column round, central canal round. 



Geological Position, etc. — Only known from the Devonian of 

 America. 



' Mon. Rec. and Foss. Crin. ,p. 6, proposed for species of Platycrinus 

 "with central valvate unobtrusive mouths, or mouths capable of being 

 withdrawn into the visceral cup." 



