of the Anal Plate in Antedon. 301 



It is not necessary to suppose, as Dr. Clark does, that the 

 posterior pafaradial represents a phylogenetically persistent 

 anal x, which subsequently is reduplicated in the other 

 interradii. • 



The facts (riven by Dr. Clark seem to me to show that 

 the parafadials of the Promachocrinidae are really of radial 

 origin. There is no more reason to regard them as inter- 

 radials than there would be in the case of the Catillocrinidae. 

 A similar proliferation of arm-bearing, or, rather, brachiole- 

 bearin" 1 , elements is seen in many Cystidea Rhombifera, 

 e. g., Cheirocrinus (Bather, 1013, Trans. R. Soc. Edinburgh, 

 xlix.' p. 446, figs. 52-55). 



Dr. Clark, it is true, has some ingenious remarks, appa- 

 rently intended to show that there is no real difference between 

 a radial and an interradial. To quote from his paper of 

 1912 (p. 312) : " while a plate if situated belowthe ventral 

 edo-e of the perisomic surface may give rise to a simple series 

 o£ more or less similar plates running up to the edge of the 

 ventral surface, and possibly continued further along the anal 

 tube, the same plate if situated just at the ventral surface will 

 c-ive' rise to an arm or a group of arms exactly like those 

 arising from the radials. The character of the ossicles 

 following a plate is not determined so much by the character 

 of the plate itself as by its position in reference to the 

 boundary between the dorsal and ventral surfaces of the 

 animal." This seems to me hopelessly inconsistent with 

 well-known facts. On the one hand, there are plenty of 

 o-enera in which the anal is on a line with the radials and yet 

 does not give rise to an arm (e. g., Hexacrinus) ; on the 

 other, there are genera in which some radials, though on^ a 

 level 'with the others, cease to bear arms (e. g., Trlbrachw- 

 crinus, Sycocrinus). , 



We have, then, found no reason for accepting Dr. Clark s 

 statement that a " representative of anal x actually occurs 

 along with the plate which he calls the radianal. Conse- 

 quently that argument for the radianal nature of the latter 

 plate falls to the ground. The facts do, however, throw 

 light on the non-migration of the anal in Promachocrinus, the 

 peculiarity which, it was admitted, might conceivably en- 

 courage one to regard it as the radianal (antea, p. 298). 

 Dr. Clark says (p. 337) : " I have examined pentacrinoids of 

 Promachocrinus b'rquelensis in which both the radianal and 

 anal x are present, the former dwindling, the latter increasing 

 in size. They are situated side by side between the. two 

 posterior radials," Obviously the lateral growth of the 



