44 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



skeleton is strong and devoid, or nearly so, of accessory ossicles. 

 They are ponderous in Hudsonaster, Palseaster, Australaster, Devon- 

 aster, Xenaster, Trimeraster, Petraster, etc. When the smaller 

 accessory plates are introduced, the inframarginals also reduce in 

 size; this development is best seen when passing from the heavily 

 plated Hudsonaster to the more abundantly plated Ilesopalseaster 

 and finally Promopalseaster. The next step in this phyletic line 

 would be pronounced a cryptozonian because the infra- and supra- 

 marginals could not be distinguished from the abundance of acces- 

 sory ossicles. In fact, it is not always easy to make out the various 

 categories of plates even in Promopalseaster, and here one must 

 resort at times to a study of the tips of the rays where the seven pri- 

 mordial columns are not yet disturbed by the introduction of accessory 

 pieces. 



In primitive Hudsonaster, where the inframargmals are compara- 

 tively very large, there are only half as many of these ossicles as 

 there are of adambulacrals, but in the great majority of Paleozoic 

 genera of Phanerozonia this marked difference tends to be less 

 decided and toward equalization. In certain phyla, as, for instance, 

 from Hudsonaster to Mesopalxaster and Promopalxaster, the equal- 

 ization has gone on rapidly, in fact, in but a short part of Ordovicic 

 time. In more conservative stocks, as from Hudsonaster to Devonic 

 Xenaster and Trimeraster, the equalization i? far slower, as is seen 

 in the following figures: Hudsonaster, 12 inframargmals and 24 

 adambulacrals; Trimeraster, 14 and 23; and Xenaster, 20 and 32, 

 respectively. In nearly all the genera these columns of plates alter- 

 nate with one another. This of course must be so where the number 

 of ossicles is different in the two columns, and complete alternation 

 is retained even in certain species where the numbers of ossicles are 

 alike, as for instance in Promopalseaster spinulosus. On the other 

 hand, in Mesopalxaster sJiaJferi, where there are 16 inframarginals 

 and 18 adambulacrals, the plates alternate in the proximal half of 

 the ambulacra and are opposite one another distally. A study of 

 the growing ray tips shows that the various ossicles all appear 

 practical!}^ of one size and where there are more adambulacrals than 

 inframarginals, the former are crowded or ally. The latter develop- 

 ment appears to be the primitive type of growth, but this mode is 

 rapidly changed to one in which the seven columns of primordial 

 ossicles appear in cycles or rings. This type of growth is easily made 

 out in primitive forms such as Hudsonaster and Mesopalxaster, and 

 in some species of Promopalxaster. However, when accessory plates 

 are developed in abundance, and especially in forms where the seven 

 primordial columns of plates remain of one size, their introduction 

 in cycles can only be made out in very young individuals or at the 



