88 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
ated faces. Those of the fixed brachials form irregular ridges, which proceed 
from both sides of the plates toward the middle without meeting. The 
apposed faces of the free arm plates have been rarely observed, but so far as 
known they are more or less concave, and we have found on some of them 
indications of transverse ridges. The lines of union between the brachials 
generally have a waving outline, and many of the species have a sharp pro- 
cess projecting from the distal end of the plates, which fits into a shallow 
depression upon the outer face of the plate below. In some species, especi- 
ally the larger ones, this projection forms a separate plate. The arms of all 
Ichthyocriuide are uniserial, the joints quadrangular, and they are destitute 
of pinnules. 
Tl. THE PLATES OF THE ACTINAL SYSTEM. 
A. The. Orals. 
Tue orals are not always represented in the adult Crinoid. When 
present, they surround the mouth or cover it; and they may occupy the 
whole face of the ventral disk, or only its median portions. In the former 
case they rest upon the edges of the radials; in the latter against the peri- 
some. In Crinoids with a regular pentamerous symmetry they consist of five 
pieces, interradially disposed, and occupy the centre of the disk. When the 
symmetry is irregular, they are pushed more or less toward the anterior side. 
The former condition prevails among the recent Crinoids, and in the Larvi- 
formia; the latter is the general rule among Paleozoic forms. When asym- 
metrical, the posterior oral is pushed in between the four others, and is 
generally larger (Plate III., Figs. 11, 17, 18, 20, 21, 22, and 23). 
The orals are among the earliest plates developed in the larva. They 
make their appearance simultaneously with the basals, upon which they rest 
until the radials are introduced, when they occupy the beveled upper edges 
of the latter plates. In the larva of recent Crinoids they form a pyramid, 
composed of five nearly equal pieces, which at first are laterally united and 
closed at the top; but they soon open out, and expose the tentacular vesti- 
bule. Ata more advanced stage the orals are carried inward by perisome, 
until finally in most of the species they become resorbed, and are replaced 
by upward perisomie growth. In only a comparatively few recent forms do 
they persist through life, and in these cases they occupy the median portions 
of the disk, and enclose the oral opening (Plate III., Figs. 9 and 10). 
