MUSEUM OF COMPAEATIVE ZOOLOGY. 131 



Lyman * gives two very instructive figures of the young Hemipholis 

 cordifera, from which he draws the following conclusion : " It thus ap- 

 pears that radial shields so nearly universal among Ophiurans are not 

 special plates, but entirely homologous with other disk-scales, and by no 

 means the first to appear." In the younger of these two stages of the 

 young Hemipholis there is a dorsocentral surrounded by five radial plates 

 and an outer circle of five interradial, while there are ten arm-joints. 

 The development of the basal plates must be very much more retarded 

 in this genus than in Amphiura ; for in the latter, with half the number 

 of arm-joints and many more interradial plates, the radial shields and 

 some other plates have appeared. The second figure (Fig. 11, op. cit.) 

 of a young Hemipholis older than the last is very interesting. In it 

 there is a dorsocentral surrounded by five radials. In each of the inter- 

 radii there are three interradial plates. There is a second radial beyond, 

 abaxiaUy to the primary radial. Peripherally placed to the second radial 

 are radial shields, one on each side of the radius. The condition here 

 is about the same as that which we find in an Amphiura in which the 

 development of the arms is very much less (Fig. 19) than in Hemipholis. 

 Every observation would probably agree with the above-quoted statement 

 of Lyman, that the radial shields are not the first to appear ; and it 

 is thought that they are the same in mode of origin as the dorso- 

 central, radialia, or interradial plates. The radial shields arise before 

 the underbasals, which are the only other plates in the radii in this 

 early condition. They are the first of the radial series to arise abaxiaUy 

 to the radialia. 



2. Plates op the Actinal Region op the Disk. 



The plates of the mouth originate early in the development of the 

 young Amphiura. The so-called V-shaped plates {ad}, ad^), described 

 below as the first and second pair of adambulacral, are among the first 

 to form, and can be seen as trifid calcareous bodies in the bilaterally 

 symmetrical embryo (Figs. 7, 8, 10). Although I have not directly fol- 



* Report on Challenger Ophiuroidea, PI. XL. Figs. IT, 12, p. 157. As stated 

 elsewhere, the young Ophiurans clinging to the arms of Hemipholis (Ophiolepis) 

 were observed by Stimpson many years ago. 



The younger stages in the formation of the plates in the young Hemipholis would 

 offer an interesting subject for investigation for those naturalists who work in locali- 

 ties where it is found, as from a comparison of Lyman's figures of the young with 

 those here given of Amphiura it is suspected that there is considerable variation in 

 the two genera in this particular. 



