80 THE VOYAGE OF H.M.S. CHALLENGER. 
almost entirely limited, is presented by the four species Pentacrinus miilleri, Pentacrinus 
maclearanus, Pentacrinus wyville-thomsoni, and Pentacrinus alternicirrus. The nearest 
approach to Pentacrinus asteria is to be found in Pentacrinus miillert (Pl. XVII. fig. 9), 
as might be expected for various reasons. The arm-groove is narrower, but the covering 
plates which rest on its edges pass up on to the pinnules alternately from opposite sides 
very much as in Pentacrinus asteria ; though the successive groups do not overlap one 
another so much as in that type, and there is more differentiation of the side plates upon 
the pinnules (Pl. XV. figs. 7,8). In Pentacrinus wyville-thomsoni the arm-groove is still 
narrower, and the ambulacrum almost entirely withdrawn into it (Pl. XVII. fig. 4). The 
plates bordering it are smaller and more irregular than in Pentacrinus miilleri, and more 
distinetly limited to the pinnule-bearing side of the arm; while the intervals between 
the joints are larger and covered by small irregular plates as in Pentacrinus naresianus 
and Pentacrinus blakei. The plating of the pinnules is limited at first to their outer 
sides (Pl. XVII. fig. 3); but it eventually appears on the inner sides as well, and 
becomes differentiated into covering plates resting on a limestone band which is sometimes 
imperfectly separable into side plates (Pl. XVII. fig. 2). 
A further reduction in the width of the arm-groove and in the size of the plates at its 
edges appears in Pentacrinus alternicirrus (Pl. XXVII. fig. 6). The intervals between 
successive joints which are occupied by the muscular bundles are larger than in Penta- 
crinus wyville-thomsoni, and are more distinctly plated. The rudimentary covering 
plates are limited to the origins of the pinnule-ambulacra, and a short distance behind 
them ; so that between every two pinnule-ambulacra of one side there is a short space of 
unprotected arm-groove. As in Pentacrinus wyville-thomsoni, the bases of the pinnule- 
ambulacra are plated on the outer side only, and in their distal portions the lateral band 
on which the covering plates rest is not divided into side plates (Pl. XXVII. fig. 5). 
Lastly, in Pentacrinus maclearanus the arm-groove is extraordinarily narrow, and 
bounded by little else than the broad plate-like upper surfaces of the component joints 
(Pl. XVII. fig. 1), while the covering plates are almost entirely limited to the pinnules 
(Pl. XVI. figs. 2, 3). They are relatively small, and the lateral band supporting them, 
though broad at first, soon narrows away considerably. 
The disk of Metacrinus presents much the same variations in the extent to which it 
is plated as that of Pentacrinus does. In Metacrinus nobilis (Pl. XLIII. fig. 3) there is 
a tolerably continuous pavement with well defined ambulacral ridges. These are bounded 
by about four rows of plates, those of the two inner rows being transversely elongated, 
and alternating with one another. In other types the anambulacral plates are more 
isolated as in Pentacrinus decorus, being more closely set, however, along the sides of the 
ambulacra, which are covered by longish plates. This is the case in Metacrinus angulatus 
(Pl. XXXIX. fig. 2), Metacrinus cingulatus, and Metacrinus nodosus (Pl. L. fig. 2). 
The scattered arrangement of the anambulacral plates is not well represented in the last 
