184 THE VOYAGE OF H.M.S. CHALLENGER. 
and above these fifteen or twenty small plates in each depressed intertertiary area. 
Intersecondary radial areas have one rather large plate in each axil, and a dozen or more 
smaller ones filling the depressions between the tertiaries. Intertertiary areas have in 
like manner one plate in each axil, and several smaller ones above. . . . The vault is 
somewhat convex in the central part, and undulates towards each intertertiary area. It 
is composed of numerous polygonal plates. Those in the central part are the larger ones, 
and each of these bears a central tubercle, which is sometimes prolonged so as to be 
designated a spine toward the margin, or rather following the undulations toward the 
intertertiary areas, the plates are smaller and possessed of slight convexity. They unite 
in the depressions in the intertertiary areas with the plates of the calyx, or rather the 
interprimary radials graduate through the intersecondaries and intertertiaries to the plates 
of the vault without any line of separation. The plates become smaller as they approach 
the inner face of the arms, over the swelling undulations of the vault, and continuing to 
decrease in size, form a somewhat granular continuous integument, that covers the 
ambulacral furrows.” 
Except as regards the larger central plates (orocentral and orals), this description 
would apply equally well to the disk of many Pentacrinide and Comatulide. The vault 
of Glyptocrinus would appear to have been more or less flexible as in the Ichthyocrinide ; 
and the so-called “ continuation of the vault up the inner side of the arms” seems to me 
to be nothing but the extension on to the arms of the ambulacral skeleton, together 
perhaps with some of the anambulacral plates at its sides, just as in Pentacrinus asterius, 
Pentacrinus alternicirrus, Pentacrinus naresianus, and Metacrinus murrayt (Pl. XVII. 
fig) 7; PL X XVI. figs: 1:25, Pl. XXVUL fig. 135.Ph XXX. fig. 2 oR) EE hes ie). 
The vault of Retocrinus nealli is thus described by Meek’ “ Interradial areas occupied 
by numerous (70 to more than 100) small pieces of very irregular size and form, and 
without any definite arrangement. . . . Axillary areas each occupied by about fifty to 
sixty very small, irregularly arranged, unequal pieces. Vault composed of numerous 
minute pieces, generally of hexagonal form; highest on the anterior side, with a ridge 
radiating to each arm-base, and a corresponding sulcus between; opening minute, 
penetrating a small tubercle situated behind the middle, and directed backward.” 
Wachsmuth says that the plates in the median part, which probably include the 
apical plates, are somewhat larger than the rest; while he further states that “the 
peculiar depressed state of the interradial and interaxillary arez, the irregularity with 
which their plates are arranged, suggests the possibility that they were adapted to 
expansion by the animal.”? Here again, then, he admits the possibility of the “vault” 
having been pliant and flexible. Thanks to his kindness, I have had the opportunity of 
examining specimens of all three genera, Glyptocrinus, Reteocrinus, and Xenocrinus, 
while Meek gives an excellent figure of the summit in Reteocrinus nealli.® Iam sorry, 
1 Paleeontology of Ohio, vol. i. p. 35. ? Revision, part ii. p. 192. 3 Paleontology of Ohio, vol.i., pl. il. fig. 3¢. 
