96 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
those plates are completely exposed, in Fig. 14 partly covered by marginal 
pieces ; but in both of them there is at the middle of the disk a moderately 
large vacant space, which in perfect specimens is tightly closed by additional 
plates. C. brevisacculus, Fig. 2, has two rings of plates: an outer one, com- 
posed of five subtrigonal pieces, of which the posterior one is largest and per- 
forated, and an inner one, composed of five pairs of plates nearly as large as 
the former but of variable size and form, which meet in the centre, so as to 
close the mouth and peristome. The ambulacra are exposed all along the 
plates of the outer ring, but are covered by the plates of the inner. In 
C. nodosus, Fig. 3, and C. multibrachiatus, Fig. 4, the four large interradial plates 
above the radials, which in the preceding figures are wholly or partly exposed, 
are completely hidden from view by small marginal pieces. The middle of 
the disk is covered by a number of rather large pieces, even more irregular 
in their arrangement than those of C. brevisacculus. The ventral disk of Euspi- 
rocrinus spiralis, Fig. 5, has at four sides a very large, convex interradial plate, 
and at the posterior side an unusually large ventral sac, with a small madre- 
porite at its base; the ambulacra are tegminal; and the median portions of 
the disk are closed by moderately small, elongate plates, arranged in rows 
with the side pieces, which meet in the centre. Very different is the disk of 
Cyathocrinus alutaceus, Fig. 6, which has at the summit five large plates, in form 
and arrangement resembling the orals of Platycrinus. The posterior one is 
largest, subcentral in position, and pushed in between the other four. There 
are no grooves along the lateral margins of the plates, the ambulacra being 
subtegminal; but the re-entering angles at the lower end enclose five well 
proportioned radial dome plates. 
Comparing the summit structure of C. alu‘aceus with that of the Camerata, 
it is quite evident that the five large plates of Fig. 6 represent the so-called 
central plate and the four larger proximals. This was also the opinion of 
Neumayr ; but while we take all five plates to be orals, he clung to the idea 
of a central plate, and recognized six orals, assuming that,two of the radial 
dome plates represented the posterior oral. We do not see how these plates, 
which occupy the median portions of the disk and cover the mouth and ends 
of the ambulacra, can be the morphological representatives of the plates 
which in Figs. 1, 2, and 5 rest upon the radials. Neumayr took the two 
structures to be equivalent, while we believe that the plates of the former 
represent the orals, and that the latter are accessary pieces of a similar 
origin to the interradial plates of the Platycrinide. 
