178 THE VOYAGE OF H.M.S. CHALLENGER. 
Platyerinide, for it had radial dome plates of the first, second, and even occasionally of 
the third order. 
Apart from this aberrant type, however, the radial regions in the vault of the 
Platycrinidee seem to have consisted of a double row of small, more or less alternating 
plates. Their arrangement does not by any means correspond regularly to that of the 
radial calyx plates, as would appear from Wachsmuth’s generalised description of the 
vault’ in the Spheeroidocrinide, z.e., Platycrinide, Actinocrinide, and Rhodocrinide. 
In fact the greater part of this description holds good for the Actinocrinide only. 
The peripheral portion of the vault of Platycrinus, 7.e., the zone betweenthe proximal 
dome plates in the centre and the calyx interradials, is comparatively small; and its 
interradial spaces are ‘occupied by three—rarely five—plates, smaller than the central 
dome plates, and less nodose, but yet comparatively large, and resting upon the inter- 
radial of the calyx.”’ This series of four or six interradials, taken all together, doubtless 
corresponds generally to the single large interradial of Cyathocrinus, as was supposed by 
Wachsmuth when he considered the latter as an oral. I do not mean that the one plate 
is homologous to the larger number; but only that they all belong to the same system 
of interradial plates. ‘The position of the alternating dome plates in Cyathocrinus and 
Platycrinus would then be very much the same. They rest in the one case between, and 
in the other upon the interradials, and terminate against the apical dome plates. Wachs- 
muth says, for example, “in Platycrinus the interradial plates thus take exactly the 
same position as the exposed parts of the oral plates in Cyathocrinus, while the covered 
parts are unrepresented.”* In this type too the calyx interradials enter into the com- 
position of the summit, just as is the case in Cyathocrinus. Thus Wachsmuth says 
that “the first interradial, which exceptionally in this group is placed almost within the 
dome regions, is identical with the outer (7.e., primary) interradial plate of Coccocrinus,”* 
in which I entirely agree. 
He further says, “the vault of the Platycrinide differs in several particulars from 
that of the other Spheeroidocrinide, and in these same characters it approaches the 
Cyathocrinide.”’ Ido not myself think that the vault of a Platycrinite was exactly of 
the same nature as that of an Actinocrinite, 7.e., that it covered in the whole of the visceral 
mass and the ambulacra on its upper surface. For if the alternating dome plates represent 
the covering plates of recent Crinoids, as Wachsmuth suggests, then all the periphery of 
the dome, outside the apical dome plates (orocentral and orals), must be the real ventral 
surface of the body, and not a tegmen calycis as in Actinocrinus. Wachsmuth himself 
admits that the alternating plates in the dome of Platycrinus, like those of Cyathocrinus, 
are represented by the covering plates of recent Crinoids; and also that no tubular 
skeleton has been discovered beneath the vault of any Paleeocrinoid except an Actinocrinite. 
1 4mer. Journ. Sci. and Arts, vol. xiv. p. 187 ; and Revision, part ii. p. 15. * Revision, part ii. pp. 17, 30, 69. 
3 [bid., part ii. p. 30. * Tbid., part ii. p. 18. Thid., part ii. p. 16. 
