Mr. W. K. Fisher on Asteroidea. 171 



Beyond a certain similarity due to the outward form, 

 particuharly to the slender rays, the Liackiidte have little 

 claim to this genus. 



The presence of intermarginal pLites and of single am- 

 pulla, and the absence of actinal intermediate plates, and of 

 interbrachial septa with calcified pillar, will debar Metrodira 

 from the Asteropidse, Ganeriidae, and Poraniidre, while 

 additional differences in the abactinal skeleton make the last 

 two families unsuitable homes. 



Even the Echinasteridte appear to be incompatible. Tlie 

 abactinal plates of Metrodira are tessellated, not at all reticu- 

 late, while the marginal plates are large, compact, and form 

 a vertical side to the ray. The interbrachial septa are almost 

 lacking. 



It seems better to place this genns in a separate family 

 rather than to enlarge the Echinasteridte and, in a measure, 

 spoil its homogeneity. The case of Metrodira is much like 

 that of Acantliaster, Mithrodla, and Valvaster, which have at 

 one time or another been in the EchiuasfceridEe. 



Metrodiridse, new family. 



Characters. — Whole skeleton overlaid by a rather thin skin 

 partially obscuring the outlines of the plates ; rays slender ; 

 abactinal plates tessellate, partly imbricated, with small 

 spines; marginal plates large, forming side-wall of ray, there 

 being one or more series of intermarginal plates on the 

 ]n'Oximal part of ray ; actinal intermediate plates reduced to 

 one, between mouth-plates and inferomarginals ; papulte 

 isolated, strictly abactinal ; tube-feet with sucking-disk 

 devoid of calcareous deposits ; ampullse single ; interbrachial 

 septa rudimentary, practically absent. 



Priamasfer, Koehler'^. — This genus, founded on Priam- 

 nster magnificus, Koehler, is very close to Gephyreaster^ 

 Fisher t> type Mimaster sxolfti, Fislier, North Pacific Ocean. 

 Priamaster has much wider ambulacral furrows, quadriserial 

 and very large tube-feet, and small actinal intermediate areas. 

 The arrangement of spines on the moutii-plates of Gephyre- 

 aster is very characteristic, and is figured (Joe. cit. pi. Ivii. 

 fig. 2). The abactinal paxilliform plates of Priamaster, the 



* Zool, Anz., Feb. 13, 1912, p. 157 ; Deuzieme Exp. Autarctic|ue 

 Fran^aise, Echinodermes, 1912, p. 91, pi. viii. figs. 1, 3, 8. 



t Ann. Si, Mag-. Nat. Hist. ser. 8, a'oI. v., Feb. 1910, p. 171 ; Asteroidea 

 of the North Pacific etc., 1911, p. 175, pi, xxx. figs. 1-4, pi. Ivii, figs. 2, 

 2 a-c, pi. lix. figs. 5, 5 a. 



12* 



