of the Anal Piatt in Antedon. 209 



If these plates in Comactinia and Comaiilia are houx] >. 

 gous with those first observed by Thomson in Antedon bifida, 

 then they Beern to support Tliomson's interpretation of those 

 plates. Further, plates occupying such a position in the 

 adults of Palaeozoic crinoids, or in such Meaozoic genera as 

 Grttet tardier iniu and Uintacrinus, are no longer called " inter- 

 radials" but " interbrachials," and are not regarded as 

 homologous with the true interradials of such forms as the 

 Bhodocrinidae. Consequently there does not seem to be good 

 reason for regarding such plates in the ordinary comatulid 

 larva as homologous with true primary interradials. Their 

 late appearance in development also suggests that they do 

 not represent plates of former importance in the cup. Even 

 if these plates were primary interradials, they would not, in 

 my opinion, have any bearing on the anal question. Anal x 

 is a characteristic plate of the Inadunate Crinoids — in other 

 words, of those crinoids which are devoid of true interradials 

 or of any interbrachially situate cup-plates in interradii other 

 than the posterior. Whatever anal x may be, it is a special 

 plate developed or adapted for the widening of the anal area 

 and the support of the rectum. The same is the case with 

 the corresponding plate in the Adunata and the Flexibilia. 

 There is no reason for regarding it as one of five primary 

 interradials, retained while the other four have disappeared. 

 Consequently the existence of plates, whether interbrachials 

 or true interradials, in all five interradii of certain comatulids, 

 does not prevent us from regarding the specially developed 

 anal plate as the homologue of anal x. 



Dr. Clark's final argument, on which he lays most stress, 

 is drawn from Promachocrinus and Thaumatocrinus. Ever 

 since P. H. Carpenter described Thaumatocrinus renovatus in 

 1884 there has been a tendency to regard the plate in the 

 posterior interradms, which supports a short somewhat arm- 

 like process, as an anal plate. It is, however, one of five 

 similar plates, each separating the adjacent radials, and 

 therefore, so long as attention was confined to the original 

 specimen, all to be regarded as true interradials. Dr. Clark 

 still so regards them, and at the same time homologizes the 

 posterior one with anal x. That homology is open to the 

 same objections as have just been raised in the case of the 

 supposed interradials of the ordinary comatulids. 



Dr. Clark's own work, however, by putting a new com- 

 plexion on T. renovatus, has made his homology even moie 

 difficult of acceptance. He has shown, in the first place, 

 that T. renovatus is the young of the species later described 



20* 



