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 

 Ichthyocrinidae are uniserial, the joints quadrangular, and they are destitute 

 of pinnules. 



II. THE PLATES OF THE ACTINAL SYSTEM. 

 A. The Orals. 



The 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 fi\Q 

 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 Palaeozoic 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. At a more advanced stage the orals are carried inward by perisome, 

 mitil finally in most of the species they become resorbed, and are replaced 

 by upward perisomic 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). 



