of the Anal Plate in Antedon. 295 



right posterior radius. This is universally admitted with 

 regard to the radianal, which is in its origin the right posterior 

 inferradial, in other words part (if not really the whole) of 

 the radial itself. The same connection does not appear to be 

 so generally recognized in the case of anal x, but this con- 

 nection is one of the reasons that led me to give that plate 

 the name brachianal. Facts proving the connection were 

 published in the ' Geological Magazine ' for January 1899, 

 and summarized in Lankester's ' Treatise on Zoology,' part iii. 

 Echinoderma, pp. 120-122 (1900). 



The geological history of the radianal is briefly as follows: — 

 It begins in Ordovician times as an inferradial *. Then, as 

 the posterior intenadius widens, the radianal of the Dicyclic 

 Inadunata moves to the left of the right posterior radial, and, 

 while retaining its contact with the lower margin of that 

 plate and with the two underlying basals, comes also into 

 contact with anal x, if that plate be present (e. g., Palaeo- 

 crinus and Botryocrinus). In Silurian and Devonian times 

 the widening of the interradius continues, the radianal con- 

 tinues to move to the left and comes into contact with a plate 

 (rt) which sinks down on the right side of the anal tube 

 between x and r.post.R. (e.g., Euspirocrinus). This arrange- 

 ment is emphasized in such Carboniferous genera as Poierio- 

 crinus. In some later forms of the same group the radianal 

 may stretch upwards, as the body of the animal with its anal 

 structures comes to lie on rather than in the dorsal cup ; but 

 while the other anal plates (x } rt) pass up beyond the limits 

 of the cup, the radianal invariably retains that connection 

 with the lower slope of r.post.R. which bears witness to its 

 inferradial origin. This arrangement is best shown in some 

 North-American and British species of Ulocrinus, and I have 

 recently discussed them in regard to this feature j". A parallel 



* An inferradial is the lower half of a transversely bisected radial, the 

 upper half being distinguished as superradial. These terms were chosen 

 to indicate the truly radial nature of these plates, O. Jaekel and A. H. 

 Clark call the inferradials " subradials." To this term there are two 

 objections. First, it implies that the plates are not radial elements, but 

 independent plates developed below the radials. This, however, does not 

 appear to be the intention of those two authors. Secondly, the term 

 subradial was used for the basals by De Koninck, and Dr. Clark seems to 

 have forgotten (p. 104) that in this use De Koninck was followed by 

 several American writers of repute down to the end of the nineteenth 

 century. As to possible homologies of the inferradials, see Bather, 1900, 

 'Treatise on Zoology,' iii. p. 112, and 1913, Bull. Mus. Ottawa, i. 

 pp. 9, 14. 



t 1917, Trans. Geol. Soc. Glasgow, xvi. pp. 210-212. On p. 211, in 

 line 4 of the middle paragraph, " left posterior basal " is a slip tor " right 

 posterior basal." Of course, B.A never touches l.post.B. in any genus. 



