296 VERRILL 



The adambulacral plates bear a regular marginal row of three to 

 seven slender spinules, and usually two exterior longitudinal groups 

 or rows of shorter spinules, which may be angular and obtuse ; and 

 toward the tips of the rays, some of them, in the type, become larger 

 and longer, as in Tosia and Ceramaster. Some of these spinules may 

 be replaced by spinuliform or clavate, two- or three-bladed pedicel- 

 lariae. The actinal disk plates are angular, often rhombic, closely 

 arranged in rows parallel with the ambulacral grooves, covered with 

 a rosette of granules, the central granules often replaced by a wide 

 valvular pedicellaria. The dentary plates are not very prominent; 

 each has an actinal row of larger spinules, similar to those of the 

 oral margin. 



This genus closely resembles Ceramaster, as limited above. The 

 principal differences consist in the more elevated and convex abac- 

 tinal plates, especially in the papular areas, where they are more 

 widely separated by the large papular pores and united by interven- 

 ing small internal ossicles, which give them a stellate appearance. 

 On other parts of the disk, especially near the interradial margins, 

 the plates are angular and closely joined in a mosaic, as in the former 

 genus. The large valvular pedicellariae are also, to some extent, char- 

 acteristic, but the marginal, actinal, and dentary plates and their 

 spinules are essentially the same in the two genera. 



MEDIASTER ^QUALIS Stimpson. 

 Plate II, figure i; plate in, figure i ; plate v, figures 3-5 (details). 



Mcdiaster cequalis Stimpson, Journ. Boston Soc. Nat. Hist., vi, p. 490, pi. 



XXIII, figs. 7-1 1. 1857. Whiteaves, Trans. Royal Soc. Canada, rv, p. 117, 

 1887. Verrill, Trans. Conn. Acad. Science, i, p. 326, 1867 ; x, p. 179, pi. 



XXIV, figs. 10-12, 1899. Fisher, op. cit., 19116, p. 198, pi. xxxv, figs. 1-3; 

 pi. Lix, figs. I, la-ic. 



Rays five, variable in length, usually about equal to the diameter of 

 the disk, regularly tapered, slender at the tip. Radii usually nearly 

 as 1:3. Marginal plates, on each side of a ray twenty-two, above 

 and below, in a specimen having the greater radius 36 mm. The 

 plates on the margin of the disk are higher than wide, with the 

 intervening sutures somewhat oblique. The lower marginal plates 

 are similar in size and shape. All are closely covered with small, 

 rounded granules. Abactinal areas of the rays are wide at the base, 

 where they may consist of seven or nine rows of plates, but they 

 rapidly decrease to three rows, and only the median row reaches the 

 apical plate. The papular areas are large, covering nearly the whole 



