REVISION OF PALEOZOIC STELLEEOIDEA. 46 



extreme tips of tlie rays, conditions not often attained among these 

 fossils. 



In the great majority of Paleozoic Phanerozonia the inframar- 

 ginals and adambulacrals lie closely adjacent to one another. In 

 typical asterids they are not separated by the introduction of acces- 

 sory interbracMal pieces until in the Devonic. Here in Xenaster 

 and Tnmeraster isolated pieces, and finally a column of them, are 

 inserted in the proximal haK of the rays. In the large-disked asterids, 

 this separation occurs much earlier, in the Mddle Ordovicic in 

 Petraster. 



In Paleozoic Phanerozonia where the skeleton is made up of an 

 abundance of more or less equal-sized ossicles, one notices also on 

 the distal portions of the rays a tendency for the mframarginals to 

 appear on the dorsal side and not to remain on the ventral, as m 

 primitive stocte. This alteration brings the adambulacrals to the 

 margm of the rays, and is a hint as to how Phanerozonia gradually 

 change into Cryptozonia. It is best seen in Promopalseaster, Petras- 

 ter, and Mesopalseaster. In Australaster it is a marked feature, in 

 fact, one can say that the distal parts of the rays in that genus are 

 cryptozonian while the bulk of the arms are still phanerozonian. 

 This ontogenetic appearance is in harmony with phylogenetic devel- 

 opment and chronogenesis. The hving Echinaster sepositus is in 

 youth also a phanerozonian, but at maturity is a typical cryptozo- 

 nian. In so typical a cryptozonian as TJrasterella, one still fuids m 

 half-grown U. ulrichi a number of inframarginals m the interradial or 

 axiUary areas. The same retention of the first formed mframargin- 

 als is also met with among the multi-rayed Cryptozonia in HeliantJi- 

 aster, and even among the Auluroidca in Encrinaster. 



It has just been pointed out how the inframarginal columns are 

 eliminated as marginal plates. They are not, however, removed 

 from the skeleton m these early forms by resorption or by failure 

 of development, but for want of special growth. In other words, 

 the inframarginals are probably present in all Paleozoic Asteroidea, 

 but because of lack of speciahzation through the developmental 

 tendency to greater skeletal flexibility, remain small and are lost 

 as such in the mass of the dorsal plates. This apparent elimmation 

 of the mframarginals has gone on independently in various stoclvs 

 as pointed out elsewhere, and therefore the absence of large marginals, 

 either infra- or supramarginals, or both series, is not of ordinal value. 



Supramarginalia. — In Hudsonaster the promment supramarginal 

 plates of the dorsal side are placed decidedly inside of the inframar- 

 ginals, though the former clearly overlap the latter. This primitive 

 position is retained in many Paleozoic genera, and apparently not 

 before the Devonic do these two colunms of ossicles come to lie 

 whoUy upon one another, and then they together margin the animals. 



