REVISION OF PALEOZOIC STELLEROIDEA. 43 



Adambulacralia. — ^Adambulacralia are present in all Stelleroidea. 

 They are best and typically developed in the Asteroidea, more or 

 less modified in the Aiiluroidea, and completely altered into the thin 

 scalelike side plates of the Ophiuroidca. In number and relative 

 size they vary as do the ambulacralia, being least numerous and 

 largest in the most primitive genera. Originally they probably 

 bore no spines at all, the plates being practically smooth. In Hud- 

 sonaster and most Ordovicic and Siluric, and some Devonic genera, 

 their surfaces are granular to tubercular. These protuberances in the 

 younger genera may have borne small spines, but long before these 

 appeared each adambulacral bore two or more stout or slender, com- 

 paratively long spines along its ambulacral edge. 



In most of the Paleozoic genera there are in each column as many 

 adambulacrals as there are ambulacrals, and both series are as a rule 

 arranged opposite one another. In Anorthaster, however, there are 

 fewer and therefore larger adambulacrals than ambulacrals, and 

 these latter ossicles clearly alternate with one another. 



In the Phanerozonia the adambulacrals never margin the rays 

 but always lie inside of the bordering inframarginals. In the Crypto- 

 zonia, however, the adambulacrals margin the animals and here the 

 ossicles are usually small, though at times they are relatively large 

 and make a stout outer skeletal frame as m Stenaster, Tetraster, 

 and Schanaster {?) montanus. The same is true in Encrinaster 

 of the Auluroidea. 



The adambulacrals in probably all the Paleozoic Phanerozonia 

 continue into the oral region and each two adjoining columns meet 

 here in a pair of modified, elongate, pomted pieces, the most promi- 

 nent ossicles of the oral armature. No other skeletal parts lie in 

 front of these oral ossicles except in the phanerozonian Hudsonaster 

 narrawayi (pi. 1, fig, 1) and in the young of the cryptozonian 

 Vrasterella ulrichi (pi. 30, fig. 7). Both are primitive forms of 

 their respective phyla and whether these five pieces or tori are to 

 be interpreted as five spines or whether they represent five primi- 

 tive orals is not yet determinable. 



Inframarginalia. — In the great majority of Paleozoic Phanero- 

 zonia the inframarginals alone margin the animals, and only in a 

 few forms (Spaniaste7% Miomaster, Rhenaster, and Neopalseaster) 

 have the supramarginals moved outward and completely covered 

 the inframarginals, so that the two columns together equally bound 

 the rays and disk. That condition is a peculiarity common to most 

 Paleozoic phanerozonians, while the wholly superposed arrange- 

 ment distinguishes nearly all the Mesozoic, Cenozoic, and Recent 

 genera of the same kind of Asteroidea. 



The inframarginals are usually the most conspicuous ossicles of 

 the Paleozoic Phanerozonia, and thig is especially true where the 



