REVISIOlir OP PALEOZOIC STELLEROIDEA. 105 



intcrbrachial arcs, and from two to five interbrachiai marginal plates 

 in each interbrachiai area. Pi^omopalxaster is a direct development 

 from Mesopalseaster, in that it preserves the generic characters of the 

 latter, which is in turn a descendant of ITudsonaster. In Hudsonaster 

 there are no interbrachiai plates, accessory or ambital columns. In 

 Mesopalxaster the smgle axillar}^ marginal plate of Tludsonaster has 

 been crowded orally and is no longer a marginal but an axillary 

 interbrachiai. Further, the radial and both marginal columns are 

 more or less completely separated from one another by accessory 

 columns of smaller plates. In Promo palseaster this addition of 

 columns of small plates is carried to its greatest development. The 

 character, however, which is relied upon to distinguish it from Meso- 

 palxaster is that PromoixilEeaster always has two or more interbrachiai 

 margmal plates in each interbrachiai area, while the former genus 

 has one, the axillary interbrachiai, and Hudsonaster has none. The 

 position and number of axillary and interbrachiai marginal plates 

 will readil}" distmguish these genera, but if the abactinal area of a 

 new form alone is seen it will be difficult to determine whether it is a 

 Mesopalseaster or Promo palseaster. Small size and few accessory 

 columns, especially ambital, will help somewhat to distmguish 

 Meso palseaster from Promo palseaster. 



Accepting Promo palxaster as a descendant of Mcsopalxaster, it is 

 seen that the axillary interbrachiai plates of the latter have in P. 

 exculptus been followed by two proximal pieces of adjoining infra- 

 marginal columns. The same is probably true of P. spinulosus, but 

 here no axillar}'" interbrachiai has been observed. In P. spedosus 

 and P. hellulus, another pair of proximal inframarginals have become 

 interbrachiai plates, but the axillary intcrbrachial is also unknown 

 in this species. In P. dyeri and P. magnifi.cus the axillary mter- 

 brachial is present along with two pairs of intcrbrachial marginals. 

 In other words, there is in Promopalseaster a constant increase in the 

 size of the interbrachiai area produced by the crowding of pairs of 

 proximal inframarginal plates. 



P. spedosus and P. hellulus are closely related species, and so far 

 as can be determined have but two interbrachiai marginal plates, 

 while our theoretic development requires three. There is, however, 

 in each area orad to the interbrachiai plates a small hiatus which in 

 some cases appears to be filled by extensions from the adambulacral 

 plates. It seems therefore probable that the axillary interbrachiai 

 plate may be retained, but is not now^ discernible on account of its 

 small size or adhering clay; it may also have dropped out or have been 

 absorbed and its place occupied by the adjoining adambulacral plates. 



In a general way it can be stated that there is also a regular increase 

 in the number of accessory plates from Mesopalxaster to Promopalse- 

 aster through P. spinulosus, P. spedosus, P. hellulus, P. dyeri into 



