KEVISION OF PALEOZOIC STELLEROIDEA. 47 



are the radial columns of ossicles, which are especially well seen in 

 Hudsonaster, Spaniaster, Devonaster, Xenaster, CaUiasterella, and 

 less prominently in Mesopalseaster, Promopalseaster, and Petraster. 

 The prominence of the radials is lost in the same way as has been 

 described for similar reduction in the supramarginals and inframar- 

 ginals. 



In primitive genera the supramarginals and radials lie closely 

 adjacent as prominent columns, but in the gi'eat majority of Paleo- 

 zoic Asteroidea these columns are more or less extensively separated 

 from one another by the introduction of radial accessory plates. The 

 latter are always small pieces and in general one can say that their 

 number increases with geologic time and with the evolutionary 

 complexity of the various stocks. At first; the accessory pieces appear 

 singly between the plates in the proximal region of the rays, where, 

 increasing in numbers, they separate the primary columns more and 

 more and finally also crowd in between the individual ossicles, es- 

 pecially the proximal radials, pushing these apart more or less com- 

 pletely. This is true not only in chronogenesis but also in onto- 

 genesis. It is best seen in a study of the dorsal areas beginning with 

 Hudsonaster and Spaniaster, and proceeding to Mesopalxaster, 

 Devonaster, Promopalxaster, and TJrasterella. In the forms with 

 great disks the development of accessory plates is extreme, as may be 

 seen in Petraster speciosus. 



The columns of radial plates are less persistent in development 

 than any other of the primary columns. Like all of the primary 

 plates, the radials are apt to lose their ponderosity and in the course 

 of geologic time become smaller. In this way they may lose their 

 individuality in the mass of dorsal plates, but in some cases radiaha 

 appear to fail to develop tlii'oughout the greater part of the rays, 

 although one or more primordial ossicles of each radius are present 

 on the disk. In certain species of Mesopalseaster, and more especially 

 in Promopalseaster, the radialia are either not present or have been 

 squeezed out of the mid line of each radius so as to be unrecognizable. 

 In other forms of the latter genus they are well developed and aiTanged 

 in linear columns. Promopalxaster as a rule has no radialia on the 

 rays and the space between the supramarginals is occupied by many 

 small accessory radial pieces. Among the Cryptozonia, radialia are 

 usually not discernible as such on the rays, though present on the disk. 



Accessory plates. — These small pieces of irregular form are space 

 fillers between all of the primary columns of plates, with the exception 

 of the ambulacrals and adambulacrals. As the body cavity grows 

 larger, skeletal increase takes place either through enlargement of 

 the seven primordial columns of ossicles or by the intercalation and 

 formation of new pieces, the accessory plates. The former method 



50001°— Bull. 88—15 4 



