REPORT ON THE CRINOIDEA. 171 



riug of radial dome plates outside them. These proximal dome plates thus correspond 

 exactly to the orals of Symhathocrinus and Hcq)locrinus, 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, i.e., as the actinal representatives of the basals, like the correspond- 

 ing plates in SymhathocHnus. K this be admitted, it follows that the proximal dome 

 plates of all Platycrinidae, Actinocrinidse, and Ehodocrinidse are also homologous with 

 the orals of Neocrinoids. 



In most Platycrinidas 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 Actinocrinidse the correspondence between 

 the actinal and abactinal sides is very complete. Thus in Stelidiocrinus capitulum ^ the 

 orocentral and six proximals (orals, mihi) 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 o-uter 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 Marsiqyiocrinus, which is placed next to 

 Platycrinus by AVachsmuth and Springer. In Mariacrinus " the vault is composed of 

 very minute irregular pieces without definite arrangement, even the apical dome plates 

 are obscure."* In Physetocrinus only the proximal dome plates are distinguishable ; 



condition having been found by liim in the Burlington limestone. He %vas 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. ^ IhicL, part ii. p. 159. 



s Amer. Journ. Sci. and Arts, vol. xiv. p. 188. * Revision, partii. p. 116. 



