of the Anal Plate in Antedon. 301 



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

 posterior pararadial represents a phylogenetically persistent 



anal ./•, which subsequently is reduplicated in the other 

 interradii. 



The facts given by Dr. Clark serm to me to show that 

 flu' pararadials of the Promachocrinidae are really of radial 

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

 radiala than there would he in the case of the Oatillocrinidae. 

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

 bearinu', elements is seen in many Oystidea Rhombifera, 

 e.g., Chdrocrinus (Bather, 1913, Trans. R. Sue. Edinburgh, 

 xlix. p. 446, 6gs. 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 below the ventral 

 edge of the perisomic surface may give rise to a simple series 

 of 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 

 give 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* 

 genera 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., Tribrachio- 

 crinuSf 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 Promachoainus, the 

 peculiarity which, it was admitted, might conceivably en- 

 courage one to regard it as the radianal (anted, p. 296). 

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

 Promachocrinus kerguelensu 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 



