GENERAL PART. 75 



movement, of which there is no trace in the embryological development of 

 this plate. The facts here shown regarding the development of the anal 

 plate and the right posterior radial would seem to lead to the conclusion 

 that the anal plate of Comatulids, which is evidently homologous with the anal 

 X of Crinoidea inadunata and flexibilia, is really a right posterior radial, 

 which is pushed out of its place and replaced by a new, secondarily formed 

 plate, which latter ultimately assumes the shape, place, and functions of a true 

 right posterior radial. This would mean, simply, that the original right 

 posterior radial has, on account of the widening of the anal structures, been 

 split up into three parts, after a horizontal and a vertical line, the former cutting 

 off a lower part, the radianal (which has disappeared in Comatulids), the upper 

 part being divided by a vertical line into a left part, the anal X, and a right part, 

 the definite right posterior radial. 



I do not venture to maintain this rather unexpected result as definitely 

 proved, but it seems to me that the facts hitherto obtained from the ontogeny 

 of the Comatulids naturally lead to this suggestion as the true morphological 

 value of these plates. 



The plates seen in the skin of the ventral surface and between the base of 

 the arms in the young Tropiometra are probably simple perisomatic plates 

 without any special morphological value. Such plates, or other plates that 

 could possibly be interpreted as interradials or pararadials, were not observed 

 in any of the other forms studied in the present memoir. 



Infrabasalia were observed hi Tropiometra; there are only 3 of them, of 

 nearly the same size. In Compsometra serrata 4 were generally found, rarely 

 3 or even 2, never 5. In Isometra vivipara infrabasalia do not occur, and the 

 same appears to be the case with Florometra serratissima and Thaumatometra 

 nutrix. In Notocrinus there are 4, sometimes 5 infrabasalia. 



These observations, together with those previously recorded, prove that 

 there is no general rule for the presence or absence of infrabasalia in Comatulids 

 since they may be well developed or entirely absent in nearly related genera. 

 Whether also species of the same genus may differ in this regard, I doubt 

 very much; at least evidence is against such supposition. Dr. A. H. Clark 

 is not of this opinion. I may quote the following passage from his paper 

 "A new European Crinoid": 63 



"A review of the facts presented by the study of Comatulid ontogeny shows 

 that Antedon bifida, and especially A. petasus, represent a phylogenetically more 

 advanced condition than the comparatively primitive Mediterranean forms, and 

 that of these latter the Adriatic species is less developed than the one found from 

 Italy westward. Now the Adriatic form usually has 4 or 5 underbasals, and the 

 one occurring at Naples, Toulon, and Villafranca 3. No underbasals have ever 

 been found in Antedon bifida, but this is not at all remarkable, nor does it reflect 

 upon the powers of observation of the able naturalists who have studied it; for 

 if the comparatively slight specialization of Antedon mediterranea over the Adriatic 



13 Proc. U. S. Nat. Mua., vol. 38, p. 330, 1910. 

 6 



