118 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
in the tegmen, enables us to discriminate between perisomic and vault pieces; 
and this led us to inquire whether such a distinction actually exists in any 
group. 
After it was demonstrated that the ventral surface of Taxocrinus is a true 
disk, we became convinced that our views respecting vault and disk would 
have to be modified as to other groups also, or be altogether abandoned. The 
latter was done in 1892, in a paper on “The Perisomic Plates,” * in which 
we endeavored to show that the so-called “vault” of the older Crinoids 
is a modified disk. This interpretation has since been accepted by Agassiz, 
Carpenter, and Bather, who agree that the use of the term “vault” should 
be abandoned in a morphological sense. The modifications that took place 
are most apparent among the Camerata, in which there is a marked increase 
in the size and rigidity of the plates, which reached its culmination among 
Carboniferous forms. 
The tegmen of Paleozoic Crinoids is often formed into ridges, which 
diverge from near the centre to the arm bases. These ridges, which are 
best defined, and occur most frequently, among Silurian Camerata, are formed 
either by covering pieces or the interambulacral plates. Such ridges also 
occur upon the disk of the Comatule, but they are formed exclusively by 
the ambulacral plates. Among the earlier forms ridges of this kind have 
been observed in Actinocrinus quinquangularis,t Habrocrinus ornatus.t Marsu- 
piocrinus depressus,§ Marsupiocrinus radiatus (Plate VIN. Fig. 15); and 
Platycrinus symmetricus (Plate LXIX. Fig. 14); in all of which the mouth 
is closed either by the orals, or in their absence by the uppermost covering 
pieces, which interlock with those of adjoining rays. Very prominent ridges 
occur also upon the disk of Zaxocrinus intermedius (Plate III. Fig. 11), in 
which, contrary to the preceding forms, mouth and food grooves are opened 
out. The ventral structure of this species bears a remarkable resemblance 
to that of the young Platycrinus symmetricus (Plate LXIX. Fig. le); all 
that is required to convert the “vault” of this Platycrinus into a disk like 
Taxocrinus, is that its orals should be parted enough to let the ambulacra 
pass in to the centre between or over their edges. Its resemblance is equally 
striking to the recent Calamocrinus and HHyocrinus ; a slight receding of the 
posterior oral and movable covering pieces would bring the three forms sub- 
stantially into the same condition, all of which shows that the closure of the 
* Proceed. Acad. Nat. Sci. Phila., pp. 345-375. 
+ Iconogr. Crin. Suec., Plate XVI. Fig. 28. t Ibid., Plate XXVII. Fig. 5. 
§ Ibid., Plate X. Fig. 16. 
