108 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
Carpenter* held the same opinion as to the vault of the Actinocrinide, 
but he believed that the “ vault” of a Platyerinoid “ corresponds collectively 
to the orals, interradials, ambulacral and anambulacral plates of Neocri- 
noids.” He agreed with ust that the calcareous network beneath the 
“vault” of an Actinoerinus “ corresponds to the limestone particles on the 
surface of the internal casts, and represents the anambulacral plates de- 
veloped in the perisome of recent Crinoids.” He also admitted “ the com- 
plete resemblance between the ventral perisome of a recent Crinoid and the 
upper surface of the body beneath the vault of an Actinocrinus.” “The vault 
and ventral disk,” he said, ‘“‘are to my mind entirely distinct structures.” 
Of the vault, he said further (p. 172): “I believe the oral or actinal system 
forming the vault of Actinocrinus to have been developed on the left larval 
antemer, in exactly the same way as the apical or abactinal system is 
developed on the right ; but the oral system, instead of being limited to five 
oral plates, as in Neocrinoids, reached a very extensive development, so that 
in its completest form it represents such a parallel to the apical or abactinal 
system as is to be met with in no other Crinoid.” From these passages and 
others in the Challenger Report, it appears that Carpenter, as well as we, 
supposed that in Actinocrinus all plates of the calyx up to the arm bases were 
abactinal, and all constituting the ventral side actinal; not only the orals 
and radial dome plates, but also the so-called interradial dome plates or 
interambulacral pieces. 
We retained the above mentioned opinion until 1881; but afterwards 
our views were materially changed respecting the interradial dome plates,+ 
which we no longer regarded as actinal structures, but took to be con- 
tinuations of the interradial plates of the dorsal cup, and as fundamentally 
identical with them. In considering the matter in the first place, we had 
not overlooked the fact that in many of these Crinoids, throughout different 
groups, the covering plates of the ambulacra are exposed at the surface, and 
as such would form a part of the “ vault.” 
It is a striking fact, in the Crinoids as elsewhere, that some characteristics 
which are of the greatest importance from a morphological point of view, 
prove to be of comparatively little value for classification. This is the case 
in a marked degree with the ambulacra of the Platycrinidee and Actino- 
crinidee, which are found to be tegminal and also subtegminal. In the Platy- 
* Chall. Rep. Stalked Crinoids, pp. 172-180. 
+ Chall. Rep, Stalked Crinoids, pp. 165, 166. 
{ Revision, Part ILI. pp. 16-27 (Acad. Nat. Sci. Phila., pp. 288-249). 
