REVISION OF PALEOZOIC STELLEROIDEA. 41 



Spinosity. — In general, it can be said that spines are least developed 

 in the heavily plated Phanerozonia, more so in those forms where 

 the doi-sal skeleton is made up of small ossicles in abundance, and 

 fully in most of the Cryptozonia stocks. 



In primitive Eudsonaster, all of the plates other than the am- 

 bulacrals are more or less well tuberculatcd, and those granules do 

 not now carry spines, if they ever did. In later forms, however, as 

 Promopalseaster, they may all have borne spines, some of them of 

 large size. Small club-shaped spines occur here in considerable 

 abundance along the sides of the inframarginals and the adambula- 

 crals. In Palxaster there are brushes of deUcate spines along the inner 

 edge of the adambulacrals. In Mesoi)alxaster the ventral spines are 

 better developed, fully so in Promopalseaster, and probabl}^ equally so 

 in most post-Ordovicic forms. 



Among the primitive cryptozonian genera Stenaster and Tetraster 

 no spines are now seen, but undoubtedl}^ such were present on the 

 actinal side. In Urasterella the entire dorsal skeleton is more or less 

 spinose, consisting of long, nonarticulating, slender rods, the dorsal 

 extensions of the ossicles. In the multi-rayed Cryptozonia of the 

 Devonic, spinosity is at its greatest development in the Paleozoic. 



Ambulacralia. — Undoubtedly the most important skeletal parts of 

 the Stelleroidea are the ambulacrals. In general they undergo the 

 least alteration during geologic time of the entire asterid skeleton, 

 and therefore any marked variation must be of broad classificatory 

 value. Schondorf ^ is well aware of this fundamental value and has 

 made full use of it in defining his three "classes," Asteroidea, Aulu- 

 roidea, and Ophiuroidea. In the Asteroidea the ambulacraUa of ad- 

 joining columns are nearly always placed directly opposite one 

 another, with the podia issuing through openings that are not in the 

 plates themselves, but laterally between two adjoining ambulacrals 

 and the corresponding adambulacrals. Among the Paleozoic asterids 

 one is not always certain whether the ambulacraUa are arranged 

 ''opposite" or ''alternate," because the specimens in nearly all cases 

 have suffered more or less from distortion. In many good specimens 

 they are very slightly alternate, but in all such cases the arrange- 

 ment is said to be alternate. In other fine fossils they are now 

 alternate, but a close study seems to show that originally they were 

 practically opposite in arrangement. On the other hand, certain of 

 the species with vnde ambulacral furrows and large ambulacralia, as 

 in Promopalseaster, Anortliaster, and Urasterella, have an alternate 

 arrangement, while in other species of the first and last named genera 

 they are just as certainly opposite. The writer therefore does not 



1 Jahrb. nassauiseh. Ver. Naturk., Wiesbaden, vol. 63, 1910, pp. 20G-25C; Palaeontographica, vol. 57, 1910, 

 pp. 1-56. 



