648 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



margins. The distichals, and all the higher orders of brachials to the last 

 bifurcation, generally consist of two plates each,* of which the upper one is 

 axillary. These plates have the form of free arm plates, being rounded on the 

 back, but the lower ones are suturally connected with the plates of the disk, 

 and in some species also with the radials, or with their fellows of the 

 opposite side of the ray, and are therefore rigid. The rays divide above 

 the costals into two branches, which bifurcate independently, the axillaries of 

 both branches bearing an arm upon one side, and the next order of brachials 

 upon the opposite side, the last axillary supporting two arms. Pinnules 

 strong, the first pinnule given off in some species from the first distichal, 

 in others from the proximal arm plate. 



Ventral disk pyramidal, convex, or almost flat. Orals slightly excentric, 

 and more or less asymmetrical. Ambulacra composed of two series of cover- 

 ing pieces, which are exposed upon the disk. The interambulacral plates 

 variable in number, except the first row, which in part is interbrachial, and 

 always consists of three plates transversely arranged. The middle one of 

 these plates is larger than the two at the sides, which are elongate and bend 

 outward to form the ambulacral passage. The upper ends of the side plates 

 rest against the covering-pieces of the ambulacra, the lower ends against the 

 sides o£ the costals or distichals, as the case may be, and sometimes against 

 the palmars. The anal side has also three plates in the first row, of which, 

 as a rule, the middle one is widest and sometimes higher than the corre- 

 sponding plates of the regular sides ; these are followed by numerous small 

 plates. Anus excentric, either opening out directly through the disk, or 

 located at the top of a protuberance, sometimes at the end of a tube. 



Column of moderate length, the lower end giving off rootlets at irregular 

 intervals ; the joints elliptic and twisted, the twist being imparted to the 

 whole stem, permitting motion in all directions. The articulation between 

 the joints bifascial, there being a well defined ridge following the long 

 diameter of the joints. Axial canal very small. 



Distribution. — This genus constitutes one of the leading fossils of the 

 Subcarboniferous on both sides of the Atlantic. It made its appearance in 

 the Hamilton group, and became extinct in the St. Louis, reaching its 

 climax in the Lower Burlington limestone, from which horizon we recognize 

 twenty-eight species, not counting the numerous synonyms. 



* In P. iiwomptus, and some others, representing a transition toward Eiiclaiocrmns, the orders of 

 brachials above the palmars consist of three successive plates. 



