REPORT ON THE CRINOIDEA. 81 



mentioned figure ; and it is too small to show the very numerous water-pores piercing 

 the plates, some of which have as many as twelve or fifteen openings. They are much 

 less abundant in Metacrinus cingulatus and Metacrinus angulatus (PI. XXXIX. fig. 2), 

 while the plates- are also smaller. 



The disk of Metacrinus differs from that of Pentac^Tnus in the greater irregularity of 

 its ambulacra, the branches of which proceeding to the large lower pinnules often come 

 off directly from the primary groove-trunks, or even from the peristome itself This is 

 especially well shown in Metacrinus angulatus and Metacrinus nobilis (PI. XXXIX. 

 fig. 2 ; PI. XLIII. fig. 3). 



Another point of difference is the relatively larger size of the anal tube in Metacrinus, 

 which is well shown in Metacrinus nodosus (PI. L. fig. 2). It may occupy the whole of 

 the interpaknar area in which it lies, and is often considerably inflated, so as to be a 

 somewhat prominent object on the surface of the disk. It is erroneously represented as 

 perfectly bare in the figure of Metacrinus nodosus, and this actually seems to be the case at 

 first sight. Closer examination shows, however, that its apparent bareness is really due 

 to the smoothness and very close approximation of the plates which cover it.' They are 

 thinner than the corresponding plates in the other interpalmar areas, and form a smooth 

 continuous pavement over the whole of the lower part of the tube, becoming more 

 nodular and. irregular towards the top. The whole appearance of the anal tube in this 

 species forcibly recalls Buckland's well known figure of the " abdominal integument " of 

 Extracrimis briareus.' There are indications of this close pavement on the anal tube of 

 Metacrinus nobilis (PI. XLIII. fig. 3) ; and it is better sho'^vn in a curious specimen of 

 Metaxirinus angulatus (PL XXXIX. fig. 2), which has a smaller supplementary anal tube 

 by the side of the larger one. 



The plates of the pinnule-ambulacra in Metacrinus are better differentiated on the 

 whole than those of Pentacrinus. For in the outer parts of the pinnules, at an}- rate, the 

 covering plates rest upon a row of distinct side plates (PL XLVII. fig. 11; PL LI. 

 figs. 11, 12 ; PL LII. figs. 5, 6), and not upon an almost undivided band of limestone as 

 in most species oi Pentacrinus (PL XIII. fig. 15 ; PL XXXVII. fig. 23). 



In the lower parts of the rays and arms the anambulacral plating of the disk extends 

 outwards at the sides of the ambulacra, in which the arrangement of plates is confused and 

 indefinite (PL XLI. fig. 13). Farther out, however, where the zig-zag course of the 

 ambulacrum (still distinctly above the arm-groove) is more marked, and the ambulacra! 

 plates less abundant, the elongated shape of the plates immediately bordering the groove 

 is more distinctly visible (PL XLI. fig. 4). In most species their extremities gradually 

 become bifid, as is well shown in Metacrinus angulatus and Metacrinus murrayi 

 (PL XXXIX. fig. 13 ; PL XLI. fig. 14). Both of these, especially the former, have the 



' See the remarks on the disk of Pentacrinus icyville-thomsoni, ante, p. 76. 

 2 Geology and llineralogy, voh i. p. 439 ; voh ii. pi. 51. fig. 2. 



(ZOOL. CHALL. EXP. PART XXXIT. 1884.) li 11 



