ASTEROIDEA OF NOBTH PACIFIC AND ADJACENT WATERS — FISHER. 197 



areas; papiiljc sinfcle or jrrouped in twos or threes; marginal plates commonly not 

 tumid; actinal interradial areas extensive; adanibulacral plates with straight 

 furrow margin bearing three to eight rather slender, often prismatic spinelets, the 

 actinal surface with a rosette or double row of shorter spineiets which may be 

 angular and obtuse, and toward tips of ra3's some of (hem may become larger and 

 longer. Some of these spinelets may be replaced by spinuliform or clavate, two- 

 or three-bladed pedicellaria^. No odd median mouth spine as in Pseudarchastcr. 

 Abactinal and marginal pedicellariie, low valvate foraminate; actinal intermediate 

 pedicellariic like the abactinal, or with slender spatulate jaws. Ihulimentary 

 superambuJacral plates present. 



Remarlcs: — This genus is closely related to Ceramnster, as here limited, but 

 differs in having internal radiating ossicles connecting the slightly lobed well-spaced 

 abactinal plates, and in possessing also rudimentary superambulacral plates. 

 These two characters will also separate Mediaster from Xytnpliaster and Xereidaster 

 both of ^vIlich I have examined with reference to these features. The analytical 

 key will serve to contrast the present genus with others more distantly allied. 

 The Neciria of Gray which bears a certain resemblance to Mediaster is probably 

 not so close as the abactinal surface would lead one to expect. There are Lnter- 

 marginal papulir, superambulacral plates, and sometimes jiincer-shaped peilicellaria?. 

 with several jaws. 



The first adccjuate account of this genus was given by Verrill (1890) who ranked 

 in it the following forms: -1/. scqualis Stimpson, M. bairdii Yerrill (olim Isaster), 

 M. agassizii Verrill, M. pediccllaris Perrier (olim Goniodiscus) , M. arcuatus (Sladen), 

 M. japonicus (Sladen) M. patagonicus (Sladen). The last three species were 

 described under Pentagoneister by Sladen. A very important feature of the genus, 

 the rudimentary but constant^ present superambulacral plates, was overlooked 

 by Verrill, who was the first to discover the internal abactinal ossicles. The three 

 species of Sladen I have since examined with special reference to the presence of 

 the internal ossicles and superambulacral plates, as these forms were not accessible 

 to Verrill. Arcuatus is a Medinster, but patagonicus and japonicus lack the diagnos- 

 tic characters of the genus, and fall within the limits of Ceramaster. Both pedicel- 

 laris and agassizii have not been dissected so that they must still remain question- 

 able members of the genus, as also M.Jlorifer (Alcock). It is impossible t<> identify 

 a Mediaster from an external examination only. The following species belong to 

 Mediaster as here defined: segualis Stimpson, hairdii (Verrill), including sfcVatus 

 Perrier, arcuatus (Sladen), tenellus Fisher, transfuga Jjudwig, ornatus Fisher. The 

 additional problematical species are: p<diceUaris, agassizii, and florifir. Mediastrr 

 clegans Ludwig (19n.5) lacks the diagnostic features of the genus; there is no advan- 

 tage in placing it in the present group. 



KEY TO THE SPEaES OF MEniASTER HEREIN DESCRIBED. 



Papulae grouped in twos and threes; pedicellariae bivalved, longer than high; pcdiccllarioD of actinal 

 interradial areas like those of the abactinal surface; adambulacral spinelets rather robust and 

 prismatic squalh. p. 1!)S. 



Papula; single; pedicellarise broadly spatulate, slightly higher than wide, those of actinal interradial 

 areas with long slender jaws, unlike the abactinal; furrow spinelets long, conipressod, rather 

 delica te Iniellus. p. 202. 



