REVISION OF PALEOZOIC STELLEEOIDEA. 105 



interbrachial arcs, and from two to five interbrachial marginal plates 

 in each interbrachial area. Promopalseaster is a direct development 

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

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

 there are no interbrachial plates, accessory or ambit al columns. In 

 Mesopalseaster the single axillan^ marginal plate of Hudsonaster has 

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

 interbrachial. Further, the radial and both marginal columns are 

 more or less completely separated from one another by accessory 

 columns of smaller plates. In Promopalseaster 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- 

 palseaster is that Promopalseaster always has two or more interbrachial 

 marginal plates in each interbrachial area, while the former genus 

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

 position and number of axillary and interbrachial marginal plates 

 will readily distinguish 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 Promopalseaster. Small size and few accessory 

 columns, especially ambital, will help somewhat to distinguish 

 Mesopalseaster from Promopalseaster. 



Accepting Promopalseaster as a descendant of Mesopalseaster, it is 

 seen that the axillary interbrachial 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 axillary interbrachial has been observed. In P. speciosus 

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

 interbrachial plates, but the axillary interbrachial is also unknown 

 in this species. In P. dyeri and P. magnificus the axillary inter- 

 brachial is present along with two pairs of interbrachial marginals. 

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

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

 proximal inframarginal plates. 



P. speciosus and P. bellulus are closely related species, and so far 

 as can be determined have but two interbrachial marginal plates, 

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

 in each area orad to the interbrachial 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 interbrachial 

 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 Mesopalseaster to Promopalse- 

 aster through P. spinulosus, P. speciosus, P. bellulus, P. dyeri into 



