112 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



We have already pointed out that the covernig pieces are exposed not 

 only in the Platycrinidse, but frequently also in other families of the Came- 

 rata^ — exceptionally even in the genus Actinocriniis . 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 Flatymmis 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 Pterotocriniis, the last survivor of 

 the Hexacrinidge, 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 Platycrinidse, for it had radial dome plates of the 

 first, second, and even occasionally of the third order." In other Platycri- 

 nidse the oral system, he says,t 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 Platycrinidas is not a true vault, or tegmen eedycis, like that 

 of the Actinocrinidae, 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 Plat?jcrinus 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 



* Cliall. Rep. Stalked Crinoids, p. 177. t Ibid., pp. 179-180. 



