342 PROCEEDINGS OF THE ACADEMY OF [1888. 



jilates; while in the later Talarocrinus (PI. XVIII, fig. 7) they seem 

 to have disappeaied entirely, leaving only the central plate, from 

 which the covering plates to the ambulacra pass directly out. In 

 forms like Batocrinus (fig. 5), and Eretmocrimis (fig. 10), where 

 there is a strong, nearly central anal tube, we find the central plate 

 resting against, and forming the base of the tube, and the four prox- 

 imals pushed far over to the anterior side, and greatly displaced. 



In some forms of Platycrinus the central position of the posterior 

 plate is well marked (Rev. III. PL, VII, figs, 5, 6, 7, 8, and PI. 

 VIII, fig 6), varying somewhal in degree. Some recently acquired 

 specimens of this genus exhibit most clearly a transition from a 

 centrally located plate surrounded by proximals and anals, char- 

 acteristic of the foregoing figures, to a set of five nearly equal 

 j:)lates, occupying the center of figure in the vault, and from whose 

 five re-entering angles the ambulacra pass out to the arms, as shown 

 by the beautiful specimen in fig. 15, (and also by figs. 4, 8, and 9). 



In all these cases it will be observed that the posterior plate is in- 

 serted between the four proximals to a greater or less extent, sepa- 

 rating the postero-lateral ones, so that the five plates meet in the 

 vault in a manner substantially similar to the five plates composing 

 the ventral pyramid of Ifaplocrinus. No one who is acquainted 

 with the structure of palaeozoic crinoids will doubt that the five 

 unsynimetrically arranged plates in the vault of Dorijcrinus, Batocri- 

 nus, etc, are structurally identical with the five nearly equal plates 

 centrally located in the specimens of Platycrinus above mentioned. 

 And it will be seen at once that all the disturbance observable in 

 diflJerent degrees in these various forms was primarily caused by 

 the anal structures, which pushed the plates — especially the posterior 

 one — out of their primitive position. Regarding these five plates as 

 the orals, it will be found that the five radial-dome-plates lie within 

 the re-entering angles all around, and that the two rings of plates 

 thus correspond exactly in their relative position with the basals and 

 radials upon the dorsal side in the Crinoidea, and the genitals and 

 oculars in the Echini. 



The above interpretation of the plates meets with no serious diffi- 

 culty from a morphological point of view. The only objections 

 occuring to us that might be urged au:ainst it are: 1. that the mouth 

 would be situated beneath the posterior oral ; and 2. that some 

 species of Talarocrinus andDlchocrinus have in the summit in place 

 of five orals a single very large plate, from underneath which the 



