MONOGRArH OF JAPANESE OPHIÜROIDEA, 379 



if by " primitive " they mean " embryonal " in the strict sense. 

 But if they regard the genera in question as archetypal, I must 

 dissent. My reasons for doing so may be stated as follows. 



1. The disk of these forms is entirely or largely covered 

 ■with the primaries and radial shields, a condition observed in very 

 young and small individuals of other ophiurans. The disk structure 

 of these forms is due to the simple growth of the plates found in 

 a very young stage. 



2. In these forms, the genital slits are either entirely invisi- 

 ble or exceedingly small. The genital slits are also entirely in- 

 visible in very young and small individuals of other ophiurans. 



3. The oral papillsß of these forms are usually fused together 

 to form a single piece. This feature probably represents the stage 

 previous to the division of a common rudiment into individual 

 papillae. 



4. The arms of these forms are extremely short, consisting 

 of only a small number of joints, and are covered chiefly by the 

 lateral arm plates. This condition is also observed in very young 

 and small individuals of other ophiurans. 



5. In these forms, the tentacle pores are limited to a few 

 basal arm joints. It appears to me very difficult to look upon 

 this condition as archetypal ; it may however be embryonal. 



6. The wings of the vertebrae are in these forms exceedingly 

 thick even in the very basal arm joints. This condition may be 

 derived from what is observed in a very young stage by simple 

 growth of the vertebrae without change in the relative size of their 

 constituent parts. 



Thus, the genera in question appear to me to be paedomorphic 

 in almost all structures ; so I look upon them as neotenic forms. 

 That they can not be truly archetypal, and that embryonal i.e. 



