76 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



of the radial columns or, in other words, between the radials and 

 supramarginals . Between the inf ramarginal and supramarginal plates 

 in Mesopalxaster there are also either a few incipient accessory plates, 

 which in young specimens are wholly absent, or there is a complete 

 column of these ambital plates. The accessory and axillaiy inter- 

 brachial plates are additions to the generic structure of Hudsonaster 

 and Palxaster, and further distinguish Mesopalxaster from both. 

 This progression toward a greater number of columns of abactinal 

 plates attains its maximum in Promo palxaster and is most marked in 

 P. magnijicus, which has not less than 28 columns at the base of a 

 ray and 5 interbrachial marginal plates in each interbrachial area. 

 The ambulacral furrows, which are very narrow in Hudsonaster, are 

 likewise so in Mesopalxaster, but are wide in Promo palxaster. Spi- 

 nosity, which is practically absent abactinally in Hudsonaster, is 

 developed in Mesopalxaster and very pronounced in some forms of 

 Promopdlxaster (P. spinulosus and P. dyeri) . Tliis is apparently also 

 true for the spines of the actinal area. 



The youthful plate structure of recent species seems to be retained 

 to maturity in Hudsonaster and somewhat so also in the genotype 

 of Mesopalxaster but in Promo palxaster the central area of the disk 

 is occupied by numerous very small plates and no definite arrange- 

 ment is discernible, though this is in large part due to their displace- 

 ment through fossihzation. However, it is certain that the basal 

 radials and interradials do not increase nor maintain a relative size 

 as in Hudsonaster and in the genotype of Mesopalxaster (here 

 radials only), but must have diminished to that of the accessory 

 plates from which they are now not distinguishable. From Hudson- 

 aster through Mesopalxaster into Promo palxaster the animals are 

 constantly increasing in size, and this also continues in the species 

 of the latter genus, attaining culmination in P. magnijicus and P. dyeri. 



While the generic abactinal differences between Hudsonaster and 

 Mesopalxaster are easily made out, this is not so readily accom- 

 plished between the latter and Promopalxaster. All of the abactinal 

 generic characters of Mesopalxaster are more pronounced in Pro- 

 mopalseaster excepting that in the former the ambital accessory- 

 plates are almost always undeveloped as columns, while in the latter 

 these plates are always well developed in one or more columns. 



When the actinal area is shown, Mesopalxaster is at once distin- 

 guished from Promopalxaster in that it has but one axillary inter- 

 brachial plate in each interbrachial area, while the latter has always 

 two or more up to five ( = interbrachial marginals) in each area. 



Devonaster in some respects is closely related to Mesopalxaster 

 and may be in the line of descent from the latter. However, the disk 

 in Mesopalxaster has far fewer abactmal plates, and there are no 

 distinct interbrachial arcs with numerous small accessory plates 



