46 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



It is apparently always a rare condition in the Paleozoic, but as the 

 post-Devonic asterids are as yet little loiown, we can more accurately 

 say that the modern type of Phanerozonia is unknown before Devonic 

 time. It should be added here that complete superposition of the 

 supra- upon the inframarginalia undoubtedly took place in more 

 than one stock and at different times. Accordingly, to group aU 

 asterids with large marginaha into the "order" Phanerozonia is 

 further proof that these terms cannot be used in the phyletic sense. 

 Phanerozonia and Cryptozonia are, however, of use in a descriptive 

 way. 



The supramarginals, and the inframarginals as well, in primitive 

 forms gradually attain larger size down the columns from the tips 

 of the rays toward the disk and mouth. In other words, the older 

 the ossicle in primitive genera, the larger is its size, and in aU such 

 forms the pairs of supra- and inframarginal columns terminate in 

 single plates, the primary or basal supramarginals and the primary 

 or basal inframargmals. The latter ossicles in the most primitive 

 genera are situated in the axils of the rays and are therefore also 

 known as axillary marginals, and in the more speciaUzed genera the 

 axillaries and even pairs of inframarginals are crowded orally into 

 the interbrachial areas. The basal supramarginals in primitive 

 forms always occur prominently on the disk, in derived genera they 

 are, however, less conspicuous, and where the disk skeleton is made 

 up of small ossicles, they are generally indistinguishable in the mass 

 of plates. 



We have seen that accessory interbrachial plates in some genera 

 are developed between the inframarginals and adambulacrals, and 

 to a greater extent the same is true of the development of ambital 

 accessory pieces between the former columns and the supramarginals. 

 In probably all cases where the rays and disk grow wider the addi- 

 tional space is largely covered in by the development of accessory 

 plates. As it is in the disk area, and therefore necessarily in the 

 basal portion of the rays as well, that the main increase of body 

 extent takes place, here are developed the greatest number of these 

 pieces. Rarely are the ambital accessory pieces crowded in between 

 these columns beyond the proximal third of the rays. This is best 

 seen in Devonaster and Xenaster, but in the large-disked forms, as 

 Petraster, they extend farther out and are present to within the 

 outer third of the rays. Ambital accessory plates may also be 

 present in Mesopalxaster, but are more definitely so in Promopdlae- 

 aster (especially in P. magnificus) , and in both genera they occur 

 throughout the greater part of the rays. 



Radialia. — In many Paleozoic genera of Phanerozonia and even 

 among some of the Cryptozonia, a more or less prominent column of 

 ossicles is seen down the center of the dorsal side of the arms. These 



