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



murrayi, Metacrhms costatus, Metacrinus nodosus, and Metacrinus wyvillii the disticlial 

 pinuules are flattened as well as their successors, and all have wide basal joints, as shown 

 in PI. XLVIII. 



The lower pinnules of Pentacrinus do not present anything like the characters of 

 those of Metacrinus, the only approach to this type being the relatively large size of 

 their lower joints in Pentacrinus asterius (PI. XIII. fig. 1) and in some forms of 

 Pentacrinus millleri (PL XV. fig. 3) ; while the distiehal pinnules never receive their 

 ambulacra direct from the peristome or from one of the five primary ambulacra of the 

 disk, as do the radial and lower distiehal pinnules of Metacrinus (PI. XXXIX. fig. 2 ; 

 PL XLIII. fig. 3 ; PL L. fig. 3). This character i& at once sufiicient to separate the disks 

 of the two genera ; but it does not produce any important efi'ect upon their external 

 appearance in the same way as do the large number of radials and the great size of 

 the pinuules borne by them. There are other very striking features, however, which 

 considerably aflect the general facies of the species of Metacrinus. There is always a 

 much greater length of the terminal portions of the arms which have undeveloped pinnules 

 than in any species of Pentacrinus; and this gives a curious rat-tailed appearance to the 

 general plume of arms. Compare Pentacrinus (Pis. XL, XV., XVIIL, XIX., XXV., 

 XXVIIL, XXXL, XXXIV.), Metacrinus (Pis. XXXVIIL, XL., XLII.^XLIV., XLVIIL, 

 XLIX., LL, LIL). 



An examination of these Plates will also bring out the more striking difi"erences in the 

 characters of the stems of the two types. The cirri of Pentacrinus vary considerably in 

 length and in appearance, being long and stout in Pentacrinus asterius and Pentacrinus 

 miilleri (Pis. XL, XIV.) ; shorter in Pentacrinus ivyville-thomsoni, though stiU stout 

 (PL XIX.) ; and slender in Pentacrinus naresianus, Pentacrinus decorus, and Penta- 

 crinus hlahci (Pis. XXVIIL, XXXL, and XXXIV.), the number of joints varying from 

 twenty to fifty. In Metacrinus, however, the mature cirri nearly always have over forty 

 joints, which are generally moderately stout ; though of course they are not absolutely so 

 large in the less robust types like Metactinus costatus (PL XLIX.) and Metacrinus 

 nodosus (PL L.), as in Metacrinus angidatus (PL XXXVIIL), Metacrinus cingidatus 

 (PL XL.), and Metacrinus murrayi (PL XLIL). In the three types last mentioned, and 

 also in Metacrinus inierrujAus (PL LIL), the cirri about the tenth or twelfth node are 

 larger than those below them ; but in other species this difi'erence is not so manifest. 

 All the species of the genus, however, have the older cirri more or less directed upwards, 

 as is especially well shown in Metacrinus angulatus (PL XXXVIIL), Metacrinus wyvillii 

 (PI. XLVIIL), Metacrinus interruptus (PL LIL), and Metacrinus tuherosus (PL LIII.) ; 

 though it is less marked in Metacrinus varians (PL XLIV.). This character occurs in 

 no Pentacrinus excepting Pentacrinus unjville-thomsoni (PL XIX. fig. 1), and is by no 

 means constant in that type. But the result of it is that the supra-nodal joint takes a 

 considerable share in the formation of the cirrus-socket, being more or less deeply incised 



