GENERAL PART. 73 



appears to be valid enough, a fact of no small interest and affording proof 

 that this division of the great Comatulid group is in conformity with their 

 natural relationship. 



The anal plate of Comatulids has of late been maintained by Dr. A. H. 

 Clark to represent the radianal (Monograph of the Existing Crinoids, page 

 331), while it was hitherto supposed to be homologous with the anal X of 

 Crinoidea Inadunata and Flexibilia. Quite recently Dr. F. A. Bather has 

 published a paper on "The homologies of the anal plate in Antedon," 60 in 

 which he reaches the result that there is no real support of a homology 

 between this plate and the radianal. Dr. Bather then maintains his old 

 view, that it is the anal X. The observations recorded in the present 

 memoir have some bearing on this question. 



The anal plate was found to appear before the right posterior radial in 

 Compsometra, Isometra, and Florometra, and apparently also in Tropiometra; 

 in the latter the radials have not yet appeared in the most advanced of the 

 Pentacrinoids available. In Thaumatometra the first appearance of the anal 

 plate could not be made out with full certainty. In the three first-named 

 the anal plate lies exactly in the radial midline, the right posterior radial forming 

 to the right of it, more or less distant from the radial midline, which place it 

 gradually occupies, pushing the anal over to the left side, against the adjoin- 

 ing oral. The mode of formation of these two plates is well seen in plate 

 xm, figure 2 (Compsometra), and plate xxvn, figure 3 (Florometra). The 

 named radial appears to be always the last of the 5 radials to develop; the 

 order of appearance of the other 4 radials could not be established; in fact, 

 there seems to be no such definite order of their appearance. 



In Antedon bifida the anal plate is stated by Wyville Thomson (op. cit., 

 p. 540) to develop "upon the appearance of the second and third radial 

 joints." 61 Finding this statement rather remarkable in view of the observa- 

 tions recorded here, I have examined some Pentacrinoids of this species also, 

 the result being that it appears, as in the species named above, before the 

 radials, lying in the radial midline, the right posterior radial forming to the 

 right of it. 



It is seen that this is exactly the same way in which the anal and right 

 posterior radial develop in Promachocrinus kerguclcnsis, according to the 

 description given byA.H. Clark in his " Monograph of the Existing Crinoids," 

 page 332. 



Having found this, I re-examined the Pentacrinoids of Hathrometra pro- 

 lixa, naturally expecting that the same facts would obtain here, the more so 

 as also in another, unidentified, Pentacrinoid (from the Philippines) the anal 

 and right posterior radial were found to develop in exactly the same way. 



60 Annals & Magaz. Nat. History, ser. 9, vol. i, pp. 294-302, 1918. 



61 W. B. Carpenter (op. cit., p. 727) says only that "between two of the radials, and on the same level 

 with them, an unsymmetrical plate early shows itself, the subsequent relation of which to the vent proves 

 it to be an anal plate." 



