REPORT ON THE CRINOIDEA. 159 
on the summit of the radial pentagon. Although I have examined a large number of 
individuals with especial reference to this point, I have not succeeded in finding any 
trace of an orocentral plate. At the centre of the upper edge of each radial is a minute 
opening which leads inwards beneath the dome ; but there is no arm-facet corresponding 
to this opening, still less a first brachial resting on the radial as is often found in 
Haplocrinus. The dome is rounded and smooth, and not marked by any radiating 
furrows like that of Haplocrinus, so that the ambulacra must have passed in beneath it 
over the upper edges of the radials. Why then may we not suppose them to have done 
the same in Haplocrinus ? 
In the larger specimens of Al/agecrinus the orals are smaller relatively to the radials, 
the upper edges of which have minute semicircular arm-facets; while the ambulacral 
openings above these facets are relatively larger. In the next stage of development the 
orals are still more reduced relatively to the radials, which bear distinct articular facets 
for the attachment of the brachials by means of muscles and ligaments around a perforated 
transverse ridge, just as in any recent Crinoid. Even in these individuals, however, 
which must have had fairly well developed arms, the relatively small oral pyramid is still 
closed, just as in the early Pentacrinoid and in Haplocrinus. 
Another form which remained permanently in the same condition, but had even better 
developed arms, was Symbathocrinus. For the so-called “ apical dome plates ”* (which I 
regard as orals) rest directly upon the upper edges of the articular faces of the radials ; 
and they form a closed pyramid or dome with five radial or ambulacral openings, one 
between every two orals. While, however, the orals of Allagecrinus form the whole 
dome, its centre is occupied in Symbathocrinus by a single orocentral plate, around 
which the orals are grouped, just as in Haplocrinus. For the knowledge of this import- 
ant fact and permission to make use of it here, I am again indebted to Mr. Wachsmuth, 
who will shortly describe it more fully. He is, I believe, disposed to agree with me in 
considering the central plate as an orocentral, and the circle of apical dome plates around 
it as orals, homologous with those of Haplocrinus, Allagecrinus, and the Pentacrinoid. 
This dome of oral plates in Symbathocrinus is only very rarely found preserved ; but 
its discovery by Wachsmuth is of extreme importance in many ways; while it indicates 
that although no dome has been met with in the two largest specimens of Allagecrinus, 
its absence may be only accidental and not natural. 
On the other hand, there is the possibility that the dome of oral plates in Allagecrinus 
became separated from the radials by the growth of intervening perisome, just as the 
orals are in all recent Crinoids except Holopus ; though whether they also separated from 
one another so as to open the mouth to the exterior, must remain undecided for the 
present. It is of course possible that they may have separated from one another without 
being removed from close proximity to the radials, just as is the case in Holopus; and 
1 Revision, part ii. pp. 17, 67. 
