PROMOPAL^ASTERID^. 79 



which has not less than twentj-eight columns at the base of a vay, and five inter- 

 brachial marginal plates in each interbrachial area. The ambulacral furrows, 

 which are very narrow in Hudsonaster, are likewise so in Mei^opalmaiiter, but are 

 wide in PromopaJseaster. Spinosity, which is practically absent abactinally in 

 Hndsonaster, is developed in Mesopcdseaster, and very pronounced in some forms of 

 Promopalgeaster (P. sjnnidosus and P. dyer't). This 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 Tludsonaster, and somewhat so also in the genotype 

 of Mesojmlssaster, but in Promopalgeaster the central area of the disc is occupied by 

 numerous very small plates and no definite arrangement is discernible, though this is 

 in large part due to their displacement through fossilisation. 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 Mesopalseaster (here radials only), but must 

 have diminished to that of the accessory plates from which they are now not dis- 

 tinguishable. From Hudsonaster through Mesopalseaster into Promopalgeaster the 

 animals are constantly increasing in size, and this also continues in the species of 

 the latter genus, attaining culmination in P. viagnificns and P. di/eri." 



Care must be taken, however, to avoid the conclusion that any of the known 

 species of the Promopalasasterinse descended from known species of the Meso- 

 palgeasterinse. The Promopalseasterinse appear among the earliest known Asterozoa 

 as highly specialised forms, and die out at the end of the Ordovician period. The 

 contemporaneous Mesopalgeasterinae are small and not highly specialised. It is 

 not until we reach the Devonian that the largest and most highly specialised 

 Mesopalgeasterinae appear. Nevertheless, the evidence is very strong that " Pro- 

 tnopalxaster " passed through a " Mesopalxaster " stage. 



We may also notice at this point the complication of the mouth-parts of 

 certain species of Promopalgeaster, e. g. P. elizx (p. 95), and its parallel in 

 Mesopalseaster complicatas (p. 88). This is a feature which appears peculiar to 

 the subfamily, and in my opinion serves more than any other character to establish 

 the close genetic relationship of the forms included in it. 



The subfamily Anorthasterinae is defined by Schuchert (85, p. 125) as follows : 

 *' Aberrant Promopalaeasteridae with the axillary and interbrachial areas composed 

 entirely of adambulacral pieces." It contains one genus and one species, Anor- 

 thaster miamiensis, Miller. I am very doubtful if this form has any relationship 

 with either the Mesopalaeasterinje or the Promopalseasterince. 



Sub-family MesopaL/Easterin.e, Schuchert. 



" Primitive Promopalaeasteridae with the interbrachial areas small, composed 

 of one pair of inframarginals, single axillary interbrachials [an odontophor], and 

 the adambulacral plates." 



