REPORT ON THE CRINOIDEA. 175 
the interradials. Wachsmuth! expresses this by saying that “the arms are recumbent 
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 Cyathoerinus and the Blastoids.. For not only 
ambulacral plates, but arm-joimts 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 ever the grooves 
between the interradials of Cyathocrinus ; but 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 Pentacrinide (Pl. XIII. fig. 16; 
BISXVilli fics 2,6) G3 Pl XMVIL 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 Platycrinide 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 with the 
calyx radials, like those in the Actinocrinide. Wachsmuth appears to have been in 
much doubt about their nature, and to have had considerable difficulty in making up his 
mind. For he has described them in very different terms at different times. The 
following general description was written by him as applying to both Platycrimide and 
Actinocrinidee, as well as to the Rhodocrinide. 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 interbrachial plates between corresponding brachial 
plates above the arms.” 
This description, although true of most of the Actinocrinidz, 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 different genera of the family. The vault of Coccocrinus has 
been sufficiently discussed already. That of Cordylocrinus is not known. The radial 
dome plates of Culicocrinus are as yet unknown, 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 different. 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 
1 Revision, part i. p. 91. 2 Op. cit., tab. xxvii. figs. le-lg ; tab. xxix. figs. 75d, 76a. 
8 Amer. Journ. Sci. and Arts, vol. xiv. p. 187. This passage appears again with a slight alteration in the Revision, 
part li. p. 15. 
+ Revision, part ii. p. 64. 
