352 PROCEEDINGS OF THE ACADEMY OF [1890. 



subtegminal, and this, which was accepted by leading authorities,, 

 led us to assume that in all Camerata the true disk was covered in 

 by a vault. In fact this seemed to be corroborated by the nature of 

 the plates, which, although varying considerably in size and number,, 

 in all these Crinoids are arranged on the same general principle,, 

 forming in all of them a compact rigid test, and in all of them 

 mouth and food grooves being perfectly closed. 



We have already stated that in some species of Platycrinus the am- 

 bulacra make their appearancenot at the margins of the summit plates, 

 but at some point between the orals and the arm bases, from beneath 

 the upper ring of interradials. In these species, applying Carpen- 

 ter's interpretation, the lower interradials would be perisomie for 

 they enclose the ambulacra, and the upper ones vault plates because 

 they do not. In Pterotocrinus, the last survivor of the Hexacrinidae, 

 the vault, as Dr. Carpenter admits (p. 177), "seems to have had a 

 closer resemblance to that of Adinocrinus than is the case in most 

 Platycrinidae, for it has radial dome plates of the first, second, and 

 even occasionally of the third order." Such radial dome plates, he 

 supposed, existed also in some Platycrinidae, and he asserts " There 

 was a membranous disk, the radial regions of which were traversed 

 by the ciliated food grooves beneath the ambulacral skeleton above ; 

 while the inter-palmar regions supported the interradial plates of 

 the vault." In the Actinocrinidae, however, he thought, the tegmen 

 was further extended so as to cover the wliole ventral surface. 



We never imagined that Platycrinus had anything but a membran- 

 ous disk, but we thought that the disk was continued underneath 

 the interradial plates all the way to the arm bases. Neither did we 

 suppose there were any further plates above the food grooves but 

 the alternating pieces ; nor that the latter were true vault plates as 

 Dr. Carpenter on p. 179 seems to have inferred we did. We 

 held that, while in the typical Adinocrinus the interradial dome 

 plates meet over the ambulacra, and form more or less elevated 

 ridges upon the surface, the " vault " of the Platycrinidae, by opening 

 out, exposed the covering pieces, and these were gradually incorpor- 

 ated into the test. In a typical Platycrinoid the covering pieces are 

 so modified as to lose alnaost altogether their original character, being 

 as large and nearly as heavy as the surrounding plates, and they are 

 united with the latter, and with one another, by close suture. In 

 some of the later Platycrinidae the covering pieces even may have 



