REPORT ON THE CRINOIDEA. 179 
If his first admission be correct, as I believe it to be, one would not of course expect to 
find a subtegminal ambulacral skeleton in Platycrinus.' 
There is some point on the actinal side of every Crinoid where the food-grooves leave 
the oral system covering up the peristome in which they originate, and are only closed 
by the covering plates at their sides. 
In the recent Hyocrinus this closure takes place on the disk, where the food-grooves 
come out from under the oral pyramid (PI. VI. figs. 1-4). In Symbathocrinus the oral 
pyramid was closed by the orocentral plate, and not open as in Hyocrinus; while the disk 
must have been even smaller than in that type, as the orals rest directly on the radials, 
so that the ambulacra of the arms which were protected by covering plates, commence 
directly from the sides of the oral pyramid almost as in Thaumatocrinus (Pl. LVL. fig. 5). 
The Bolland Platycrinus which was figured by Miiller is essentially in the same condition 
as Symbathocrinus, the orals (apical dome plates) resting directly against the plates of 
the abactinal side, in this case the calyx-interradials ; while the oral or actinal system 
is increased by the development of the radial dome plates corresponding to those in the 
calyx, which rest directly over the arm-openings and are followed by the ambulacral 
plates of the free rays. 
If the orals of Thawmatocrinus formed a closed pyramid resting directly on the inter- 
radials, as it must in earlier stages of growth ; and if this pyramid were further extended 
at its base by the development of radial plates in the actinal system, then the ambulacra 
would start from the periphery of these plates just as the alternating plates of the free 
rays do in Platycrinus. 
In other Platycrinide the oral system seems to have been still larger, having 
secondary and tertiary dome-radials ; but sooner or later it came into contact with the 
alternating series of plates which I take to be the skeleton of closed ambulacra, that 
perhaps only opened to the exterior at the origins of the arms from the free rays. There 
was a membranous disk, the radial regions of which were traversed by the ciliated food- 
grooves beneath the ambulacral skeleton above ; while the interpalmar regions supported 
the interradial plates of the vault. Both the ambulacral skeleton and the interradial 
1 By this I mean distinct covering plates such as are found beneath the radial dome plates of the Actinocrinide. 
There must, of course, have been a “tubular passage beneath the vault,” the presence of which is indicated on the 
natural casts from a cherty bed in the Upper Burlington limestone, which have been recently examined by Mr. Wachs- 
muth. From what he has told me about these internal casts of Platycrinus I imagine that they show very much what 
he has already described in similar siliceous casts of Actinocrinus, viz., “elevated rounded ridges, almost like strings 
overlying the surface ”; and his remarks upon these last (Amer. Journ. Sci. and Arts, vol. xiv. p. 120) seems to me to be 
equally applicable to the Platycrinus-casts. He says: “The position of the string-like ridges (in case they represent 
passages as I can hardly doubt) is analogous with that of the open food-grooves of recent Crinoids.” In Actinocrinus, how- 
ever, he not only found this evidence of passages beneath the vault which lodged the food-grooves ; but he also discovered 
in some specimens preserved in a different way, that these passages were protected by a distinct ambulacral skeleton, itself 
below the radial dome-plates. I imagine that this subtegminal skeleton, which corresponds to the ambulacral skeleton 
on the disk of Pentacrinus (Pl. XVII. fig. 6) does not exist in Platycrinus. For the ambulacral skeleton of this type 
was largely developed and external, forming the “ alternate plates of the dome ” (August 1884). 
