NOTES ON ECHINODERM MORPHOLOGY. 313 



(adambulacrals, ad 5 .) appear about the same time. 1 It is 

 'evident therefore from fig. in that the basals (3) reach a com- 

 paratively large size before the appearance of either inter- 

 radials (I.), under-basals (2), or radial shields, and that the 

 latter do not appear till after the formation of two at least of 

 the under-basals and interradials. 



From Fewkes's descriptions and figures, however, it would 

 seem that the radial shields appear much earlier in the 

 American variety, and that the developmental sequence is 

 altogether different from that described by Ludwig for the 

 European form. Fewkes's account is a little difficult to follow, 

 owing to a want of precision in his terminology. Thus, for 

 example, he three times uses the names " basals " and " inter- 

 radials" as if they were synonymous; though every previous 

 writer upon the calyx of the Crinoids and its relation to that of 

 other Echinoderms has regarded the interradials and basals 

 as fundamentally distinct in their morphological relations. 



The earliest plates to appear in the inter-radial areas are the 

 orals (figs, i, in, 5) which, as Fewkes himself describes on p. 

 128, are " forced to the actinal surface of the disc before the 

 interradials arise." He speaks of the latter, the first inter- 

 radials, as forming " on the periphery of the abactinal hemi- 

 some on interradii between contiguous radialia. They are 

 triangular in shape, and occupy a triangular interspace 

 between adjoining primary radials." He calls them, however, 

 abaxial basals or abaxial interradials, and they are designated 

 in his figure by the letters r p~. (fig. iv, I). But as they are 

 situated on the periphery of the abactinal hemisome outside 

 the closed ring of radial primaries, it is altogether incorrect to 

 speak of them as " basals/' This term is only applicable to 

 the ring of interradial plates which are situated adaxially to 

 the primary radials, i. e. between them and the dorsocentral 

 (figs, ii, in, 3). The basals of a monocyclic Crinoid, as 

 implied in their name, which was given to them by Johannes 



1 The reader will do well to remember that the first two pairs of adambu- 

 lacrals are on the ventral surface of the disc at this stage, and are there- 

 fore not visible in its dorsal aspect. 



