REPORT ON THE CRINOIDEA. 79 



A somewhat different type of ambulacrum is presented by two other Caribbean 

 species, Pentacrlnus asteria and Pentacrinus decorus. The arm-groove of the former 

 is rehitivcly wide and the proximal j)arts of the ambulacra are distinctly above it, 

 though they gradually sink do^\^a 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 UTegular double row of large plates (PI. XIII. fig. IG ; PI. 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 fi'csh 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 ^ {i.e., 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 aU fused into a narrow band of limestone 

 which rests on the pinnide-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 XXXIII. 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 

 hlakei (PL XXXIII. fig. 3). In the middle and outer parts of the arms the ambulacra 

 are generally like those of Pentacrinus asteria, though 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 theii- 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 indi\T.dualised ; 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 PI. XVII. fig. 3 ; PI. XLVII. fig. 11. 



