96 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



those plates are completely exposed, in Fig. 1 1 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. hrevisacculiis, Fig. 2, has two rings of plates: an outer one, com- 

 posed of ^\Q subtrigonal pieces, of which the posterior one is largest and per- 

 forated, and an inner one, composed of fi^Q 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. miiltihraehiahis, 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. hrevisacciilus. 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. Yery different is the disk of 

 Gyaihocriniis ahtaceus, Fig. 6, which has at the summit fi^Q large plates, in form 

 and arrangement resembling the orals of Flatycrimis, 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 ^nq well 

 proportioned radial dome plates. 



Comparing the summit structure of C. alutaceiis with that of the Camerata, 

 it is quite evident that the fiwe 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 Platycrinidse. 



