108 NATURAL HISTORY BULLETIN 



tions, which serve to unite the two subfamilies very closely. 

 Therefore it is now hard to draw the line between them. 



It may be just as well to abandon the subfamily and unite the 

 genera under Groniasterinae, as Professor Fisher (1911&) has 

 already done. 



As there is but a single species, within my present limits, that 

 can be referred with certainty to the MecHasterince, it is not my 

 intention to discuss the question at this time, and therefore I 

 now retain the group chiefly as a matter of convenience. 



Genius Media ster Stimpson. 



Mediaster Stimpson, Journ. Boston Soc. Nat. Hist., vol. vi, p. 490, pi. 23, 



figs. 7-11, 1857. Type M. cequalis Stimp. 

 Mediaster Sladen, Voy. Challenger, Zool., vol. xxx, pp. 263, 752, 1899. Ver- 



rill, Kevision Genera, p. 178, 1899. Fisher, op. cit., 1911b, p. 196. 



"Verrill, op. cit., 1914a, p. 295. 

 Isaster Verrill, Proc. U. S. Nat. Mus., vol. x\'ii, p. 257, 1894. Type M. 



bairdii Ver. 



Form stellate, with a broad flat disk and moderately long 

 tapered rays. 



The dorsal plates or parapaxillae are regularly arranged in 

 radial rows, of moderate size, somewhat elevated in the center, 

 mostly roundish, covered with a rosette of short, obtuse spinules 

 or granules. When these are removed the plates on the central 

 part of the disk and along the median region of the rays appear 

 as roundish or oval convex bosses. They are connected together 

 by five or six internal radiating ossicles, between which are the 

 pores for the papulae. Thus the plates appear to be stellate at 

 the base, though they are not actually of that shape. In a lim- 

 ited interradial area the plates are closely joined and tesselated. 

 The median row of dorsal plates extends to the apical plate of 

 the rays in the type, but not in some of the other species. Some 

 of the plates bear a central, broad, sessile, valvular pedicellaria, 

 which, in the type species, is nearly as wide as the plate. They 

 are sometimes lacking. The papulae may be single, or (as in the 

 type) clustered. 



Marginal plates are well developed, not swollen, granulated, 

 rather numerous, higher than broad, paired, upper and lower 

 series nearly equal in size and number, and with their sutures 



