MUSEUM OF COMPARATIVE ZOOLOGY. d fey | 
Lyman * gives two very instructive figures of the young Hemzpholis 
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, 
abaxially 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 abaxially 
to the radialia. 
2. PLATES OF THE ACTINAL REGION OF 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, Pl. XL. Figs. 11, 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. 
