REPORT ON THE CRTNOIDEA. 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 (PL VI. figs. 1-4). In Symhathocrinus 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 pp-amid almost as in T/iaumatocrimis (PL LVI. fig. 5). 

 The BoUand Platycrinus which was figured by Miiller is essentially in the same condition 

 as Symhathocrinus, 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 ra5^s. 



If the orals of Thaumatocrinus 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 Platycrinidse 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 



^ By this I mean distinct covering plates such as are found beneath the radial dome plates of the Actinocrinidse. 

 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 y4c<inom?M(«, 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 Platycrintcs-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 Adinocrinus, 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 e.xternal, forming the " alternate plates of the dome " (August 1884). 



