REPORT ON THE CRINOIDEA. 171 
ring of radial dome plates outside them. These proximal dome plates thus correspond 
exactly to the orals of Symbathocrinus and Haplocrinus, covering in the peristome and 
resting against the calyx plates, which in the Platycrinus are the interradials, and not 
the upper edges of the radials, as in the simpler forms, 
The disk underneath was therefore larger, owing to the greater width of the cup, and 
an additional ring of plates, the radial dome plates, was necessary to protect it. 
Wachsmuth admits the homology of these with the calyx radials, which are separated 
from the dorsocentral by the basals; and I cannot therefore see what other view can be 
taken of the proximal dome plates which immediately surround the orocentral, than to 
regard them as orals, z.e., as the actinal representatives of the basals, like the correspond- 
ing plates in Symbathocrinus. If this be admitted, it follows that the proximal dome 
plates of all Platycrinide, Actinocrinidz, and Rhodocrinide are also homologous with 
the orals of Neocrinoids. 
In most Platycrinidee the primary radial dome plates are succeeded by others of a 
different character, which do not precisely correspond to the various orders of calyx 
radials, as will be mentioned later; but in the Actinocrinidze the correspondence between 
the actinal and abactinal sides is very complete. Thus in Stelidiocrinus capitulum? the 
orocentral and six proximals (orals, mhz) take up almost three quarters of the summit, 
the remainder of which is occupied by a single anal, and three radials to each ray, 
the two outer ones being very small. All gradations can be traced from this condition 
into the complex vault of a Strotocrinus, which is described as follows by Messrs. 
Wachsmuth and Springer :— 
“The disk is paved by many hundreds of small minute pieces which decrease in size 
towards the arms, and which at the outer points of the rays become almost microscopic. 
The apical plates are larger, and are separated from each other, but not otherwise 
distinguished from the other plates, and hence are not easily identified.”’ In young 
individuals, however, there are comparatively fewer summit plates than in the adult. 
“The apical and principal radial pieces are larger than the intervening interradial plates 
which, exceptionally in this genus, attain by age the same size as the apical and 
radial pieces.”* The same kind of complexity and irregularity as in Strotocrinus, 
though not quite so well marked, is characteristic of the summit of Mariacrinus, 
Periechocrinus, and Physetocrinus, and also of Marsupiocrinus, which is placed next to 
Platycrinus by Wachsmuth and Springer. In Mariacrinus “the vault is composed of 
very minute irregular pieces without definite arrangement, even the apical dome plates 
are obseure.”* In Physetocrinus only the proximal dome plates are distinguishable ; 
condition having been found by him in the Burlington limestone. He was kind enough to send me one of these for 
examination ; and it was only after seeing it that I was struck by Miiller’s figures, to which I had previously not paid 
very much attention. I have since examined several similar specimens from the Bolland limestone. 
1 Revision, part ii. p. 99. 2 T[hid., part i. p. 159. 
3 Amer. Journ. Sci. and Arts, vol. xiv. p. 188. * Revision, part ii, p. 116. 
