ate THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
We haye already pointed out that the covering pieces are exposed not 
only in the Platycrinidx, but frequently also in other families of the Came- 
rata, — exceptionally even in the genus Actinocrinus. If it were true that in 
Crinoids in which the disk ambulacra are subtegminal, the integument which 
covers them is a “ vault,’ and, on the contrary, in those with ambulacra ex- 
posed, or “external,” a disk, it seems to us that the two forms should be 
separated as distinct orders, and it would follow that our present classi- 
fication of the Crinoids is arbritrary and worthless. These considerations 
produced in our minds a firm conviction that the integument in both cases 
must represent the same thing, being either a vault or a disk, — the plates 
either all vault pieces or all perisomic. 
In some species of Platycrinus the ambulacra make their appearance, not 
at the margins of the summit plates, but at some point intermediate between 
the orals and the arm bases, from beneath the upper ring of the interradial 
plates. In these species, applying Carpenter's interpretation, the lower inter- 
radials would be perisomic, for they expose the ambulacra; but the upper 
ones vault plates, because they do not. In Pferotocrinus, the last survivor of 
the Hexacrinidx, a very highly differentiated form, the tegmen, according to 
Carpenter,* “seems to have had a closer resemblance to that of Actinocrinus 
than is the case in most Platycrinide, for it had radial dome plates of the 
first, second, and even occasionally of the third order.” In other Platyeri- 
nid the oral system, he says,} sooner or later came in contact with the 
alternating series of ambulacral plates. “There was a membranous disk, 
the radial regions of which were traversed by the ciliated food grooves 
beneath the ambulacral skeleton above ; while the interpalmar regions sup- 
ported the interradial plates of the vault.” Further on he explains that 
the vault of the Platycrinide is not a true vault, or teymen calycis, like that 
of the Actinocrinide, but corresponds collectively to the orals, interradials, 
ambulacral and anambulacral plates of Neocrinoids; contrary to the vault 
of Actinocrinus, which not only covers in the food grooves themselves, but 
also their skeleton of alternating plates, together with the origin of the 
ambulacra and the plated interpalmar areas of the disk. 
We never imagined that Platycrinus had anything but a membranous 
disk, which, we thought, was continued underneath the interradial plates 
all the way to the arm bases. Neither did we suppose it had any further 
plates above the food grooves than the alternating pieces; nor that the 
* Chall. Rep. Stalked Crinoids, p. 177. + Ibid., pp. 179-180. 
