MORPHOLOGICAL PART. 91 



surrounding it. He regarded the former as the actinal representative of the 

 dorso-central or terminal plate of the stem, and the latter the representatives 

 of the basals. He took the proximals to be the orals, believing with us that 

 the posterior oral was divided into two plates. He said : * '' The proximal 

 dome plates rest directly against the calyx interradials, that on the posterior 

 side being represented by two small plates with the anus between them; 

 while there is a more or less tubercular ring of radial dome plates outside 

 them. These proximal dome plates thus correspond exactly to the orals of 

 BymhathocrimiB and Haplocrinus^ covering in the peristome, and resting against 

 the calyx plates, which in Platycrimis are the interradials, and not the upper 

 edges of the radials, as in the simpler forms. ... I cannot see what other 

 view can be taken of the proximal dome plates which immediately surround 

 the oro-central, than to regard them as orals \ i. ^., as the actinal representa- 

 tives of the basals, like the corresponding plates in Bymbathocrimis. If this 

 be admitted, it follows that the proximal dome plates of all Platycrinidae, 

 Actinocrinidae and Ehodocrinidge are also homologous with the orals of Neo- 

 crinoids." t Carpenter's oral theory was based almost entirely upon the 

 hypothetical oro-central, — a plate before unknown in Echinoderm morpho- 

 logy, — and the six proximal plates, which he assumed to be orals, although 

 their morphological relations had never been established. 



The same view of the question was also taken by Etheridge and Car- 

 penter,$ and afterwards by Neumayr ; § while Zittel || supposed the orals 

 to be unrepresented in all Platycrinidae, Actinocrinidae, Ehodocrinidas, and 

 Calyptocrinidae \ though admitting their presence in some of the other 

 groups. 



The above theory was laid aside by us in 1885, when we ascertained that 

 the two smaller proximals, which we had supposed to represent the posterior 

 oral, occupy a radial position, and therefore could not be orals. The struc- 

 ture shows that these plates undoubtedly represent the two posterior radial 

 dome plates, pushed to a position among the plates of the proximal ring by 

 the anus, the three anterior ones retaining their position within the re- 

 entering angles of the four larger proximals. This discovery was annoimced 

 by us in Part IH of the Eevision, p. 47, and we designated the respective 



* ChalL Rep. Stalk. Crin., pp. 170 to 171. 



f Dr. Carpenter's views on tliis subject are fully set forth in the Chall. Rep. Stalk. Crin., pp. 158 

 to 184. 



J 1886. CatalogueoftlieBlastoidea, pp. 66to75. 

 § 1889. Die Stamme des Thierreiches, p. 448. 

 11 Haudb. der Paleeont., I, p. 332. 



