110 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



two median columns are larger and more convex than the three 

 columns immediately on each side, while the pieces of the fourth 

 column are again more pronounced than those on each side of it, and 

 probably represent the supramarginals. Between the supramarginals 

 and the inframarginals are from one to three columns of ambital 

 plates, and between these are inserted here and there some small 

 extra pieces. The extra pieces are mainly developed beside the supra- 

 marginal columns. Plates of the disk like those of the rays, but their 

 arrangement is unknown. 



Madreporite unknown. 



Inframarginal plates very convex, about as wide as long except- 

 ing near the base of the columns where they are about twice as wide 

 as long. There are about 31 of these ossicles in a column, and they 

 have numerous small, slender, sharp spines articulating on well- 

 developed tubercles. 



Adambulacral plates greatly resembling the inframarginals. 

 They are largest near the mid-length of each ray, diminish in size 

 distally, but proximally they hold their width and decrease in length 

 and therefore are more numerous. There are 43 plates in each 

 column besides those in the oral armature. Ambulacrally each 

 adambulacral plate has a short attenuate extension which articulates 

 with the crests of the ambulacral plates. Between all the adam- 

 bulacral ossicles there is inserted in the suture facing the ambulacral 

 groove a single large, thick spine, excepting for a short distance 

 proximally where such are inserted in every other suture. The 

 lateral surface of the adambulacral plates in the distal half of the 

 columns bears numerous small spines like those of the inframarginal 

 series, but proximally these spines are gradually displaced by two 

 or possibly more larger ones like those on the ambulacral face. 



Ambulacral furrows wide and deep. Ambulacral plates of adjoin- 

 ing columns opposite, two or three times as wide as long, and slightly 

 overlapping proximally. Upon each plate there is a high median 

 ridge which arises at the inner end of the plate and abuts against 

 the short extension of the adambulacral plate. The podial open- 

 ings are between the plates and beneath the sutures of the adam- 

 bulacral plates, excepting near the base of the rays, where they 

 occur in every other suture. The ridges here are sigmoid and not 

 straight, as they are more distally. There appear never to be more 

 than two rows of podia in an ambulacrum. 



In each axillary area inside of two large inframarginal plates and 

 between the converging adambulacral columns are inserted two 

 pairs of wedge-shaped interbrachial marginal plates ornamented 

 like the inframarginals. 



Formation and locality. The holotype and only known specimen 

 was collected by Mr. Charles B. Dyer in the Maysvillian at Gin- 



