122 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



ginal pieces and the adambulacral, is not deceptive, it would, in 

 that respect, conform to the structure of that group, and differ 

 from Palseaster, as now understood; though I am inclined to think 

 this appearance due to the accidental displacement of the parts at 

 the point where there are some indications of a few disk pieces." 

 These pieces are not to be correlated with the accessory interbrachial 

 plates of Petraster, for in this genus accessory plates consist of two 

 short columns situated between the adambulacral and marginal 

 plates and uniting in the axillary areas. In P. dyeri, however, the 

 four interbrachial marginal plates are continuous with the infra- 

 marginal columns and evidently were derived from that series, 

 and are not accessory interbrachials. It is true that all interbrachial 

 plates increase the size of the disk, but in one they are derived from 

 the inframarginal series, by crowding and in the other they are 

 interpolated, newly developed plates that force apart the columns 

 of plates in the rays sometimes almost to the distal ends. 



PROMOPAL.EASTER MAGNIFICUS (Miller). 



Plate 21, fig. 1; plate 22, fig. 1; plate 23, figs. 1-3. 



Palseaster magnificus MILLER, Journ. Cincinnati Soc. Nai. Hist., vol. 7, 1884, 

 p. 16, pi. 4, figs. 3, 3a. 



Original description. "The diameter or breadth of the disk is 

 one and one-fourth niches, and the distance from the point of one 

 ray to the point of the opposite one, if the rays were wholly preserved 

 in the specimen under examination, would be fully 6 inches. * * * 



"The plates upon the dorsal side are very convex, and part of 

 them, at least, were spine-bearing, though it would seem that there 

 was not more than one spine upon any single plate. The arrange- 

 ment of the plates on the dorsal side of the rays is very ornamental. 

 A single series of highly convex or conical plates, larger than the 

 others, and each evidently bearing a central spine, occupies the 

 middle of each ray; on either side near the margin of each ray there 

 is a similar series, and the two intervening spaces are filled with 

 smaller, convex plates arranged in rows which are directed diagonally 

 forward from the plates of the side series to the plates of the central 

 series, forming angles with each plate in the central series occupying 

 an angle. This disposition of the plates on the dorsal side of the 

 rays will, so far as known, serve to distinguish this species from any 

 hitherto described. 



"The plates covering the dorsal side of the body or disk have been 

 so much disturbed in our specimen that one can not correctly define 

 them. 



"The ambulacral furrows are wide. The marginal plates are 

 hexagonal, about the size of the larger plates on the dorsal side of 



