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 Cyathocrinidx, not only the ambulacral p/ates, 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 Platycrinidx 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 Batocrinide ; but upon closer examination 
it appears that the two structures are not exactly parallel. We have no 
doubt that also in Cyathocrinus, 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 Batocrinus is a supplementary 
anal. The former constitutes the wppermost 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 Batocrinus, 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 Cyathocrinide apparently have close 
affinities 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., 1892, p. 64. 
