298 Dr. F. A. Bather on the Homologies 



In the absence of figures for Promachoerinus it is impossible 

 to be perfectly clear as to the precise relations of the plate in 

 question to the right posterior radial ; but the description is 

 far from convincing me that Dr. Clark's conclusion is justified. 

 In the well-known figures of the Antedon larva by W. B. 

 warpenter and others, and in those of Hathrometra prolixa 

 Chich Clark here reproduces from Mortensen as evidence in 

 his favour, I find nothing to indicate that the anal is anything 

 other than anal x. The upward migration of the plate 

 entirely favours this view. If the anal of Antedon be not 

 homologous with the plate in Promachoerinus , then the latter 

 might possibly be the radianal, since it does not migrate 

 beyond the limits of the cup. That fact, however, is scarcely 

 conclusive, since there is a special reason for it in this genus, 

 as will shortly appear. 



Let us, then, see what further arguments Dr. Clark has to 

 offer. " Since," he writes, " the radianal is represented in 

 the pentacrinoids of the comatulids we should expect also to 

 find in the posterior interradius a second plate which we 

 could with a reasonable degree of probability identify as the 

 representative of the plate known as anal x ; and such a plate 

 actually occurs." This, of course, would be almost con- 

 clusive ; but the statement needs careful checking. 



Fiisf, consider the facts adduced for genera with five 

 radials, as in Antedon. Wyville Thomson (1865, Phil. 

 Trans, p. 540) "in one or two cases observed " in Antedon 

 bifida, " about the time of the first appearance of the anal 

 plate, a series of five minute rounded plates developed inter- 

 radially between the lower edges of the oral plates and the 

 upper edges of the basals." The fate of these plates is 

 uncertain. Thomson himself identified them with certain 

 perisomic interbrachials of the adult, but P. H. Carpenter 

 (1884, Chall. Rep. p. 40) doubted this, and regarded them as 

 true interradials, ultimately rtsorbed in Antedon, but homo- 

 logous with the permanent interradials of Thaumatocritms. 

 With these plates A. H. Clark (p. 335) homologizes five 

 plates which appear in Comactinia (species not stated) at the 

 time of formation of the fii st primibrachs (IBr,) and lie on 

 the shoulders of the radials at such a height that their upper 

 halves are between the IBij. In a single specimen of 

 Comactinia rneridionalis (p. 317, fig. 412) each such plate was 

 surmounted by two others. In Comatilia iridometriformis 

 (of rather later stage, pi. ii. figs. 528, 529) a " large rounded " 

 plate rests in each interradius above the interbrachial 

 process of the radials. 



