178 A. E. Verrill — Revision Genera and Species of Starfishes. 



Mediaster Stimpson. 



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



figs. 7-11, 1857. 

 Mediaster Sladen, Voy. Challenger, Zool., vol. xxx, pp. 263, 752, 1889. 

 Isaster Verrill, Proc. U. S. Nat. Mus., vol. xvii, p. 257, 1894. 



The original description and figure of this genus and of the type 

 species by Stimpson were incomplete and rather imperfect, so that the 

 genus has not been well understood by most subsequent writers who 

 have referred to it. I have, therefore, thought it desirable to rede- 

 scribe and figure the type species at this time. 



Form stellate, with a broad flat disk and moderately long tapered 

 rays. Marginal plates 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 more or less 

 closely corresponding vertically ; oblique in the type. No odd 

 interradial plate. Abactinal plates or parapaxillse are regularly 

 longitudinally arranged, of moderate size, somewhat elevated, 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 arms 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.* The 

 papulae may be single, or (in the type) clustered. Thus the plates 

 appear to be stellate at the base, though they are not actually of 

 that shape. The median row of abactinal plates extends to the 

 apical plate of the rays in the type, but not in some of the other 

 species. Some of the abactinal 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 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 FentMgonaster of authors. Some of these spinules 

 may be replaced by spinuliform or clavate, two or three-bladed pedi- 

 cellarife. The actinal disk-plates are angular, often rhombic, closely 

 arranged in rows parallel with the arabulacral grooves, covered with 

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

 valvular pedicellaria. The dentary plates are not very prominent ; 



* I have found these ossicles in M. cequalis and M. Bairdii. Other species 

 have not been examined as to this feature. 



