MORPHOLOGICAL PART. 115 



Zittel, and Neumayr, who all regarded the plates as orals. Plausible as this 

 theory appeared to us at first, it involves serious morphological difficulties ; 

 and in 1883, on discovering that the plates consist of seven pieces instead of 

 five, we abandoned it, in which we were followed by Carpenter. Since then 

 we have taken the plates to be interradials, although with some hesitation, 

 for they are not interbrachial, nor, strictly speaking, inter-ambulacral, but in 

 part sub-ambulacral, and sub-tegminal. In their position they resemble 

 the first interradials of Platycrinus, but the ambulacra of that genus rest 

 against the edges of the plates, and only the covering pieces are exposed. 

 In the Cyathocrinidas, not only the ambulacral plates, but also the ambulacral 

 vessels rest upon solid plates, and the small marginal plates on top of these 

 plates sustain toward the side- and covering-pieces the same relation as the 

 interradial plates of the Platycrinidae toward their covering pieces. 



The presence of three plates at the posterior side, as against one at the 

 other four, would seem to indicate that they may be homologous with the 

 first row of interbrachials of the Batocrinidae ; but upon closer examination 

 it appears that the two structures are not exactly parallel. We have no 

 doubt that also in CyathocrinuSj as in the case of Batocrinus, the posterior 

 plate is broken up into two pieces by the middle plate ; but this plate in 

 Cyathocrinus is a madreporite, while that of Batocrmus is a supplementary 

 anal. The former constitutes the uppermost plate of the posterior inter- 

 radius, being separated from the first anal plate by the full length and 

 depth of the sac; the plate of Batocrmus, however, which rests directly 

 upon the anal, represents the lowest plate of the area. 



Obscure also are the relations of the small marginal plates, which in 

 Cyathocrinus overlie the larger ones, and which occur only at the four smaller 

 sides of the disk, leaving the surface of the madreporite at the fifth side ex- 

 posed to view. The only plausible explanation we can find for this structure 

 is that these plates represent the higher disk plates, which for want of space 

 overlapped, and gradually covered the larger ones ; while those of the pos- 

 terior side, instead of overlapping, were carried upward, and formed into a 

 sac or tube. 



The large interradial plates of the Cyathocrinidae apparently have close 

 aflinities with the deltoids of the Blastoidea, and Carpenter and Bather* 

 proposed to apply also to the plates of the former temporarily the term 

 "deltoids." The plates of both groups rest upon the radials and support 



* Ann. and Mag. Nat. Hist., 1893, p. 64. 



