of the Anal Plate >'n Antedon. 297 



also does its proximal supporting plate ,r, which thus grad- 

 ually rises above the radial circlet, until the two posterior 

 radials meet beneath it (e. g., Erisocrinus) . Thenceforward, 

 from the Trias onwards, the identity of anal x i.s lost in the 

 general plating of the adult anal tube when such a structure 

 exists. 



Apart from all hypothesis, the historical facts make it 

 clear that anal x and tho radianal are structures differing in 

 origin and development and ultimate fate. While anal a 

 has supra-radial relations, those of the radianal are inferradial. 

 While anal x finally passes up out of the cup, the radianal is 

 resorbed when within the cup limits. 



If, then, in recent crinoids a plate is observed in young 



2jes between the posterior radials, and then migrating 

 upwards beyond the limits of the cup, it is natural, on the 

 theory of recapitulation, to regard that plate as x rather than 

 as the radianal. 



Let us now consider Clark's arguments. 



First, as to the anal of Antedon and similar forms. He 

 says this is more closely connected with r.post.B. than with 

 l.post.R. But this is just what I have alwa\ d on 



as the case with anal x in the Ordovician and Silurian In- 

 adunata. Jf this tendency of the Antedon anal to keep to 

 the right M is a fact of the very highest importance," then I 

 welcome it as confirming the views I have always expressed 

 concerning the true anal x. 



nk then passes to the anal in the young of Promacho- 

 crinus (p. 332). First he describes it as arising "in the 

 rhombic area between the corners of the basals and orals" 

 before any of the radials appear. That is to say, it lies in 

 the right posterior radius. " Soon afterward the [r.post.] 

 radial appears, just to the right of and in line with''' the anal 

 plate. This radial grows faster than the anal and gradually 

 "surrounds" it, so that the anal ''comes to lie in a deep 

 concavity in the side of the radial." Later the r.post. 

 "radial extends itself beneath the anal and the concavity 

 becomes straightened out and disappears, the anal con- 

 currently being shoved diagonally forward (to the let t) and 

 disappearing by resorption." In a subsequent paragraph it 

 is added that the concavity which receives the anal is in "the 

 lower left hand portion of the radial," also that the anal 

 migrates upwards, 



These facts, says Dr. Clark, " leave no room for doubt that 

 the so-called anal of the pentacrinoid larvae is nothing more 

 nor less than the radianal of the fossil forms." 



.1////. & Mag. JS. Hist. Ser. 0. Vol i. 20 



