REVISION OF PALEOZOIC STELLEEOIDEA. 



35 



ossicles, other than those of the disk, tlic interbrachial areas, and the 

 accessory pieces, are introduced in all the columns at the distal ends 

 of the rays, we are permitted, in theorizing as to the characters of the 

 radicle, and from what has been learned from Iludsonaster, to elimi- 

 nate all of the rays with their ventral and dorsal ossicles do^vn to 

 the disk. If this is done, there finally results a tiny depressed and 

 mammillatcd spheroid, made up dorsally of 11 primary plates, and 5 

 margmal axillaries, and ventrally of a limited number of adambu- 

 lacrals and ambulacrals around the mouth. To make this matter 

 clearer, two diagrams are presented of the probable characters of the 

 direct ancestor of Eudsonasier (see 

 figs. 1 and 2). 



By taldng away the raj^s down to 

 near their bases, i. e., to R2, and 

 correspondmgly decreasing the size 

 of the primordial supramargmals, we 

 get the dorsal structure of a late stage 

 of the probable typembryo of the 

 Stelleroidea, as illustrated in fig. 3. 



If aU of the ray plates are com- 

 pletely removed, we apparently get 

 the typembryo of the class Stelle- 

 roidea, as illustrated in figs. 4 and 

 5. It should be stated here, how- 

 ever, that the number of the adam- 

 bulacrals and ambulacrals in figure 

 5 is entirely conjectural, though 

 these plates and those of the mouth 

 frame are among the first to develop 

 in hving forms. 



If the developmental views just stated are correct, it follows that 

 the five branches of the water- vascular and nerve rings of the typ- 

 embryo of Stelleroidea grow outward along the radii, developing as 

 theygi'ow the rays or arms with tlicir dorsal and ventral skeletons. 

 In other words, the growing tips of the radii spread outward beyond 

 the disk, and each one gives rise to nine columns of ossicles (one 

 radial, two supramarginal, two inframarginal, two adambulacral and 

 two ambulacral) . The radial plates continue as simple columns, while 

 upon the primordial supramarginals and the axillaries rest pairs 

 of columns, the supramarginals and inframarginals, rcspectivel3^ 



In Echini, however, the growing tips of the radii do not spread 

 beyond the disk (apical disk of oculars and genitals) but remain 

 internal and give rise to new ossicles, the ambulacrals (same ossicles 

 in Stelleroidea) and interambulacrals ( = adambulacrals of Steller- 



FiG. 3.— Dorsal view op theoretic typem- 

 bryo OF Stelleroidea at the beginning 



OF RAY DEVELOPMENT. THE PRIMORDUL SUP- 

 RAMARGINALS here HAVE ON EACH SIDE THE 

 BASAL PIECES OF TEN SUPRAMARGINAL COL- 

 UMNS. CD, centro-dorsal; R1, primordial 

 radial; E2, SECOND radial; Sml, dorsal in- 



TERRADU.L OR PRIMORDIAL SUPRAMARGINAL; 

 SS, BEGINNING OF SUPRAMARGINAL COLUMNS. 



