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 Allagecrinus 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 stiU 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 smaU 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-caUed "apical dome plates"^ (which I 

 regard as orals) rest directly upon the upjier 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 beheve, 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 aU 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. 



