80 THE VOYAGE OF H.M.S. CHALLENGER. 



almost entirely limited, is presented by the four species PentcLcrinus miilleri, Pentacrinus 

 maclearanus, Pentacrinus wyville-thoTnsoni, and Pentacrinus alter nicirr us. The nearest 

 approach to Pentacnnus asteria is to be found in Pentacrinus miilleri (PI. XVII. fig. 9), 

 as might be expected for various reasons. The arm-groove is narrower, but the covering 

 jilates 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 Pentacnnus wyville-thomsoni the arm-groove is still 

 narrower, and the ambulacrum almost entirely withdrawn into it (PI. XVII. fig. 4). The 

 plates bordering it are smaller and more irregular than in Pentacrinus mulleri, and more 

 distinctly 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 naresiamis 

 and Pentacrinus hlalcei. 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 weU, and 

 becomes difi'erentiated 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 occuj)ied by the muscular bundles are larger than in Penta- 

 crinus tvyville-thomsoni, and are more distinctly pLited. 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 loyville'thomsoni, the bases of the pinnule- 

 amluilacra 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 nohilis (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 Metacr^inus 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 



