REPORT ON THE CRINOIDEA. 77 
represented in Pl. L. fig. 2. It seems at first sight to be perfectly bare, but careful 
examination proves it to be covered by very closely set small plates with ill defined 
boundaries. 
The ambulacra of Pentacrinus wyville-thomsoni form rather prominent ridges, which 
are composed of four irregular rows of plates. The plates of the two inner rows are 
somewhat elongated transversely and generally closed down over the grooves, representing 
the covering plates of the pinnule ambulacra. 
A well plated disk also occurs in Pentacrinus alternicirrus (Pl. XXVI. figs. 1, 2); 
but the ambulacra are less heavily plated than in Pentacrinus wyville-thomsoni, and are 
therefore not so readily distinguished from the anambulacral plates. These are 
sometimes larger than in the Atlantic species, and are pierced by abundant water-pores 
which are not shown in the figure; but they do not always fit quite closely together, so 
that gaps of bare perisome are visible here and there. As in Pentacrinus wyville- 
thomsoni, the plates are generally larger in the anal interradius than elsewhere. 
A disk of Pentacrinus naresianus was drawn for Sir Wyville Thomson by Mr. Black 
(Pl. XXX. fig. 2) ; but it seems to have been mislaid or else cut into sections, for it has 
not come into my hands. So far as can be judged from the figure, the anambulacral 
plates were small; while the ambulacra appear to be well-defined ridges and to come 
jnto close union around a very small peristome, which is thus entirely concealed by the 
apposition of their large covering plates. 
I have not seen a disk either of Pentacrinus maclearanus or of Pentacrinus blakei ; 
but in Pentacrinus miilleri and Pentacrinus decorus it is far from being as completely 
plated as in the species already noticed (Pl. XVI. fig. 10; Pl. XXXIV. fig. 2). For the 
anambulacral plates are generally isolated and not in contact with their fellows. They 
are small and numerous in Pentacrinus miilleri; but in Pentacrinus decorus they are 
fewer in number and comparatively large, some of them containing as many as twenty 
water-pores.* 
There are about four irregular rows of plates on the ambulacra, the imner ones 
being elongated, and sometimes standing up rather prominently at the edges of the 
grooves. 
The relation of the food-groove to the arm-joints varies greatly in the different species 
of Pentacrinus, so that mere fragments of the arms can be identified by the characters 
of their ambulacra, quite apart from any peculiarities of their arm- and pinnule- 
joints. 
The middle line of the upper surface on each joint of the brachial skeleton is occupied 
by a groove of variable depth and width, to which Miiller gave the name “ arm-groove ” 
(Pl. XVIL figs. 1, 4, 7, 8, 9). It is bordered on each side by the more or less prominent 
muscle plates of the successive joints; and the vascular structures which are partially 
1 These are omitted in the figure, and the plates are drawn too close together. 
