SHALLOW-WATER STARFISHES 295 



each plate, producing a stellate structure, as seen from the inside. 

 Supra-ambulacral plates are present. Pedicellariae dorsal and ven- 

 tral, sessile, bivalve, short and broad in the type; slender, two- or 

 three-bladed ones may occur on adambulacral plates. 



Papulae mostly in radial rows, emerging from between the con- 

 nective radiating ossicles, often two or three together. 



The adambulacral spines are nearly as in Ceramasier ; distal ones 

 are larger and longer. 



The principal reason for distinguishing this subfamily is the exist- 

 ence of radiating internal ossicles connecting the bases of the 

 adjacent paxilliform plates. 



This seems to be an important structural feature, and in the case 

 of a family with so large a number of genera as Goniasteridae, it is 

 advantageous to subdivide it, if definite and important structures can 

 be found for subfamily groups, as in this case. 



Genus Mediaster Stimpson. 



Mediaster Stimpson, Joum. Boston Soc. Nat. Hist., vi, p. 490, pi. 23, figs. 



7-1 1, 1857. Verrill, Revision, in Trans. Conn. Acad. Sci., x, p. 178, 1899. 

 Mediaster Sladen, Voy. Challenger, Z06I., xxx, pp. 263, 752, 1889. Fisher, 



op. cit., 1911ft, p. 196. 

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



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 corre- 

 sponding vertically, oblique in the type. No odd interradial plate. 

 Abactinal plates or parapaxillas are regularly longitudinally arranged, 

 of moderate size, somewhat elevated, mostly roundish, covered with a 

 rosette of short, obtuse spinules or elongated 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 (as 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 abac- 

 tinal plates bear a central broad, sessile, valvular pedicellaria, which, 

 in the type species, is nearly as wide as the plate. Pedicellariae are 

 sometimes lacking. 



