178 THE VOYAGE OF H.M.S. CHALLENGER. 



Platycrinidse, 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 

 PlatycrinidfB 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 Sphaeroidocrinidse, i.e., Platycrinidse, Actinocrinidaj, and Ehodocrinidse. 

 In fact the greater part of this description holds good for the Actinocrinidae only. 



The peripheral portion of the vault of Platycrinus, i.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 («.e., primary) interradial plate of Coccocrinus,"* 

 in which I entirely agree. 



He further says, " the vault of the Platycrinidse difi'ers in several particulars from 

 that of the other Sphseroidocrinidse, and in these same characters it approaches the 

 Cyathocrinidse."^ I do not myself think that the vault of a Platycrinite was exactly of 

 the same nature as that of an Actiuocrinite, i.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 Palseocrinoid except an Actiuocrinite. 



• Amer. Journ. Sci. and Arts, vol. xiv. p. 187 ; and Revision, part ii. p. 15. ^ Revision, part ii. pp. 17, 30, 69. 



3 Ibid., part ii. p. 30. ■• Hid., part ii. p. 18. Ibid., part ii. p. 16. 



