REPORT ON THE CRINOIDEA. 307 
though Sir Wyville Thomson described a specimen with thirty arms.! The irregularity 
in the number of joints between successive axillaries is very striking, especially as 
compared with the very constant character of the ray-divisions in the Comatule. 
Taking for example the genus Actinometra, we find in Pentacrinus decorus the follow- 
ing series of distichals and palmars which are specially characteristic of different groups 
of the species of that genus. 
A. Ten arms only; the first two brachials united bifascially, and the third brachial a syzygy as in Penta- 
crinus naresianus, Group of Actinometra meridionalis. 
B. Two distichals united by a syzygy, »  Actinometra jukesi. 
C. Two distichals, the axillary not a syzygy, 3  Actinometra pulchella. 
D. Three distichals, the axillary a syzygy, »  Actinometra parvicirra. 
E. Two palmars united by a syzygy, »  Actinometra typica. 
F. Two palmars, the axillary not a syzygy, »  Actinometra stelligera. 
G. Two palmars, the axillary a syzygy, . »  Actinometra multiradiata. 
H. Three palmars, the axillary a syzygy, »  Actinometra bennetti. 
In addition to these, Pentacrinus decorus may show numerous combinations of distichal 
and palmar series such as are characteristic of other groups of Comatulze, and also certain 
conditions such as two distichals with the axillary a syzygy, which I have not as yet 
met with in any Comatula at all. These facts well illustrate what has been said above 
(ante, p. 55) respecting the difference between the arms of Pentacrinide and Comatule. 
The rays of Pentacrinus decorus and their subdivisions are sometimes in pretty close 
contact, though rarely flattened laterally ; while in other cases they are more or less 
separated by perisome (Pls, XXXV.-XXXVIL). This perisome is sometimes nearly 
bare and sometimes plated pretty continuously ; and a similar variation is apparent on the 
upper surface of the disk. This is sometimes covered tolerably closely by rather large 
plates (Pl. XXXIV. fig. 2); but the plating is not quite so continuous as in Pentacrinus 
wyville-thomsoni and Pentacrinus alternicirrus (Pl. XVIL. fig. 6; Pl. XXVI. figs. 1, 2). 
On the other hand, the gaps between the plates, though sometimes comparatively large, 
are not so extensive as in Pentacrinus miilleri (Pl. XVIL. fig. 10). The plates sometimes 
bear small blunt.spinelets which are possibly tactile in function, as they contain branches 
from the antiambulacral nerves which extend upwards on to the disk from the envelope 
of the chambered organ in the calyx (Pl. LIX. figs. 2-4, ad). The plates bordering the 
ambulacra of the disk are narrow and spine-like, often forming a kind of palisade, which 
is more distinct than in any other type of the Pentacrinide (Pl. XXXIV. fig. 2). 
They become somewhat irregular on the arm-bases (Pl. XXXII. fig. 6); but further 
out (Pl. XXXII. fig. 4) they begin to show signs of a more or less perfect 
differentiation into the side and covering plates of the pinnule-ambulacra (PI. XXXVI. 
figs. 23, 24). 
1 Sea Lilies, The Intellectual Observer, August 1864, p. 7. 
(zoOL. CHALL, EXP.—PART XXX1I,—1884,) li 43 
