184 THE VOYAGE OF H.M.S. CHALLENGER. 



and above these fifteen or twenty small plates in eacli depressed intertertiary area- 

 Intcrsecondary radial areas liave 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 Pentacrinidfe and Comatulida^. The vault 

 of Glyptocrinus would appear to have been more or less flexible as in the Ichthyocrinidse ; 

 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, Pentaco'inus naresianus, and Metacrinus murrayi (PI. XVII. 

 fig. 7 ; PI. XXVI. figs. 1, 2 ; PL XXVII. fig. 13 ; PL XXX. fig. 2 ; PL XLI. fig. 13). 



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 arese, 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, Glyi^tocrinus, Reteocrinus, and Xenocrinus, 

 while Meek gives an excellent figure of the summit in Reteocrinus nealli.^ I am sorry, 



1 Palieontology of 01ui:>, vol. i. p. 35. - Revision, part ii. p. 192. ^ Palreontologj of Ohio, vol. i., pi. ii. fig. 3c. 



