EEPORT ON THE CEINOIDEA. 175 



the interradials. Wachsmuth^ expresses this by saying that " the arms are reciuabent 

 upon the vault. There are five oral {cf. interradial) plates, upon the sutures between 

 which, and raised above the general level, the arm -joints are imbedded, being covered by 

 small alternating plates like the free arms." Here then we have a further development 

 of the abnormal condition presented by Cyatliocrinus and the Blastoids. For not only 

 ambulacral plates, but arm-joints themselves, extend over the sutures between the inter- 

 radials towards the opening at the centre of the summit. The two rows of alternating 

 plates which cover in the furrows clearly represent the plates arching over the grooves 

 between the interradials of Cyathocrinus ; l^ut it is equally clear that they are the cover- 

 ing plates of ambulacra which are borne by the arm-joints. This is very evident in some 

 of Angelin's figures" of the brachial ambulacra, which may be advantageously compared 

 with those of the ambulacra in the Comatulidee and Pentacrinidse (PI. XIII. fig. 16 ; 

 PI. XVII. figs. 2, 6, 7 ; PI. XXVII. figs. 4, 5, 11, 12 ; PL XLVII. figs. 10-13 ; PL LIV. 

 figs. 4, 7, 8 ; PL LV. figs. 3-7). 



The two rows of alternating plates in the dome of the Platycrinidse have a 

 close resemblance to those on the vault of Cyathocrinus, and I have a strong suspicion 

 that they are of the same character, and not radial dome plates homologous wth the 

 calyx radials, like those in the Actinocrinidas. Wachsmuth, appears to have been in 

 much doubt about their nature, and to have had considerable difiiculty in making up his 

 mind. For he Jias described them in very difierent terms at different times. The 

 following general description was written by him as applying to both Platycrinidae and 

 Actinocrinidse, as well as to the Ehodocrinidse. Speaking of the radial dome plates,^ he 

 says "as a general rule, the summit plates increase in proportion to the number of 

 primary arms of a species in the same manner and on the same principle as the plates of 

 the dorsal side. Every radial from the third radial upward has a corresponding plate on 

 the ventral side, and additional interbraehial plates between corresponding brachial 

 plates above the arms." 



This description, although true of most of the Actinocrinidee, does not appear to hold 

 good for any typical Platycrinoid, as far as can be judged from Wachsmuth's accounts of 

 the vault structure in the diff'erent genera of the fiimily. The vault of Coccocrinus has 

 been sufficiently discussed already. Tliat of Coixlylocrinus is not known. The radial 

 dome plates of Culicocrinus are as yet unknowTi, and but little room is left for them, as 

 the apical dome plates occupy the greater part of the summit. In the next genus, 

 Marsupiocrinus, however, the condition of the vault is entirely diff'erent. Wachsmuth 

 and Springer* describe it as follows: "vault low, hemispherical, composed of a larger 

 number of plates than usually found in this family. These are generally formed into 



• Revision, part i. p. 91. - Op. cit., tab. xxvii. figs, le-lg ; tab. xxix. figs. 75rf, 76a. 



3 Artier. Joiirn. Sci. and Arts, vol. xiv. p. 187. This passage appears again witb a slight alteration in the Revision, 

 part ii. p. 15. 



* Revision, part ii. p. 64. 



