MONOGEArn OF JAPANESE OPHIUKOIDEA. 369 



both " archetypal " and " embryonal " ; but the two must in my 

 opinion be strictly distinguished. In fact, the genera in question 

 of the Ophiolcpklidce are, in my opinion, embryonal, but not 

 archetypal. 



On the contrary, the Ophiomyxince and certain genera of the 

 Ophiacanthidce with only horizontally flexible arms, appear to me 

 to be fairly archetypal, being similar to the Palaeozoic Myophmvoida 

 in many structures, the presence of the genital plates, genital scales 

 and oral shields, and sometimes also of the radial shields being 

 the principal differences. As to the radial shields, I look upon 

 them as a modification of the marginal disk scales, often present 

 in both the Palaeozoic and recent ophiurans, which have secon- 

 darily become articulated with the genital plates where these have 

 developed ; and a very primitive condition of the radial shields. 

 may in my opinion be seen in Ophiostiba, Ophiohyalus, as well as 

 in young specimens of Ophiomyxa, &c. This view is in accord 

 with Lyman 's^\ according to which the radial shields are not special 

 plates but are homologous with the other disk scales, and by no 

 means the first to appear. 



The ventral arm plates of many genera of the Ophiomyxince, 

 as Ophiosciasma, Ophiocynodus, Ophiosysygus, Ophiolepioplax, Opkio- 

 styraciwn, Opliioliyalus, Ophiomyxa, Ophiodera, Ophiohymen, &c., 

 and some of the Ophiacanthidce, as Ophiomyces, Ophiologimus, &c. 

 are long and narrow, being in contact with, or slightly separated 

 by a naked space from, each other, at least in the proximal arm 

 joints. A similar condition may be observed also in certain repre- 

 sentatives of Palaeozoic Myophiuroida. In many genera of the 

 Ophiacanthidce, the ventral arm plates are very small, short and 



1) Eep. Challenger, V, 1882. p. 157. 



