REPORT ON THE CRINOIDEA. 79 
A somewhat different type of ambulacrum is presented by two other Caribbean 
species, Pentacrinus asteria and Pentacrinus decorus. The arm-groove of the former 
is relatively wide and the proximal parts of the ambulacra are distinctly above it, 
though they gradually sink down into it as they get farther from the disk; as long as 
the rays continue to divide their ambulacra and those of their pinnules are covered 
by an irregular double row of large plates (Pl. XIII. fig. 16; Pl. XVII. fig. 7), After 
the last bifurcation these plates become smaller and more regularly arranged, so that 
they take the form of oblong covering plates with rounded ends which stand up at the 
sides of the groove (Pl. XVII. fig. 8). They do not, however, extend uninterruptedly 
along each side of the groove, but are arranged in a series of linear groups between the 
successive pinnules of either side, so that they alternate in position on the two sides 
of the arms successively. They are largest and best developed at the base of a pinnule, 
where its ambulacrum comes off from that of the arm, and from this point they diminish 
gradually in size towards the disk until the base of the next pinnule is reached, when 
a fresh set appears upon the proximal edge of its ambulacrum. 
Thus, then, the covering plates which pass on to the pinnule-ambulacrum from that 
of the arm are at first limited to its proximal or outer side only. But a second 
set soon appears on the inner side of the ambulacrum ? (7.¢., that next the arm), and their 
outer ends gradually become more and more rounded until they present the appearance 
shown in Pl. XIII. fig. 15. Their bases are all fused into a narrow band of limestone 
which rests on the pinnule-joint and represents the side plates that are better developed 
in other species; while the rounded outer portions represent the covering plates 
proper, which alternate with one another from opposite sides, so as to leave a series of 
openings through which the tentacles are extended. 
The lower portions of the ambulacra of Pentacrinus decorus are essentially like those 
of Pentacrinus asteria, except that they sink more deeply into the arm-groove, while the 
plates covering them are smaller and far less regularly arranged (Pl. X XXIII. fig. 6). 
But the muscle plates of the successive arm-joints fit less closely together than in most 
other species of Pentacrinus, so that the muscular bundles are long and also visible 
externally ; for they are not covered in by plated perisome as in the allied Pentacrinus 
blakec (Pl. XXXIIL. fig. 3). In the middle and outer parts of the arms the ambulacra 
are generally like those of Pentacrinus asteria, thbugh not so open (Pl. XXXIII. fig. 4) ; 
for the groups of plates which protect the bases of the pinnule-ambulacra overlap 
somewhat closely from opposite sides, while their parts are more distinctly differentiated. 
Farther out on the pinnules the segmentation of the lateral limestone band is sometimes 
carried so far that the side plates can be distinctly individualised ; but there is a good 
deal of variation in this respect (Pl. XXXVII. figs. 23, 24). 
A third type of arm, with a very narrow median groove to which the ambulacrum is 
1 Compare Pl, XVII. fig. 3; Pl, XLVII. fig, 11. 
