442 
H. BURY. 
In Ophiurid Plutei the ten radial plates (primary radials 
and terminals) and one of the orals (madreporite) are early 
developed ; but the basals and most of the orals do not appear 
till the pentagonal stage is reached. In Crinoids, on the 
other hand, the ten interradials (basals and orals) are the first 
to appear, while the primary radials arise late ; no terminals 
have yet been recognised, but now that we know where to look 
for them it is not impossible that they may be discovered. In 
Asterids the terminals are usually the first plates to show 
themselves, though in Asterina the basals arise simultaneously 
with them ; in other forms the basals (except the madreporite) 
and the primary radials are late in appearing, and it is not yet 
certain whether the orals are ever developed. In Echinids the 
basals appear early, but we know nothing at present of the 
primary radials, terminals, and orals ; the ocular plates have 
usually been identified as primary radials, but some regard 
them as terminals ; embryology alone can decide this question, 
and at present my material is not sufficient for it; I am, how- 
ever, strongly disposed to believe that the oculars are ter- 
minals, and that the primary radials are entirely absent. 
The definite relation borne by the plates to the body-cavities 
is a fact of great morphological importance, and while it is 
absolutely opposed to Barrois’s statement that all the skeletal 
plates of Ophiurid, Echinid, Asterid, and Crinoid larvae are 
developed round the right enteroccel, it also throws consider- 
able doubts on Semon’s sweeping assertion (28, p. 282) that 
no homologies are to be found between the primary plates in 
the different groups. 
Another point of considerable morphological importance 
receives great light from the study of the development of the 
calcareous plates. If we look at the lateral views of Ophiurid 
and Asterid larvae (figs. 2 and 18) we shall see that the plates 
on each side may be regarded as forming a longitudinal series 
dorsal to the alimentary canal ; since those plates which are 
apparently ventral do not reach as far forward as the anus, and 
may be conceived to have reached their present position in 
connection with a general curvature of the body. The arrange- 
