342 THE VOYAGE OF H.M.S. CHALLENGER. 
murray, Metacrinus costatus, Metacrinus nodosus, and Metacrinus wyvillii the distichal 
pinnules are flattened as well as their successors, and all have wide basal joints, as shown 
in Pl. 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 (Pl. XII. fig. 1) and in some forms of 
Pentacrinus miilleri (Pl. XV. fig. 3); while the distichal 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 distichal pinnules of Metacrinus (Pl. XXXIX. fig. 2; 
Pl. XLII. fig. 3; Pl. L. fig. 3). This character is at once sufficient to separate the disks 
of the two genera; but it does not produce any important effect upon their external 
appearance in the same way as do the large number of radials and the great size of 
the pinnules borne by them. There are other very striking features, however, which 
considerably affect 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 (Pls. XI., XV., XVIIL, XIX., XXV., 
XXVIII, XXXI., XXXIV.), Metacrinus (Pls. XXXVIIL., XL., XLIL—XLIV., XLVIIL., 
DOGS, al ate): 
An examination of these Plates will also bring out the more striking differences in the 
characters of the stems of the two types. The eirri of Pentacrinus vary considerably in 
length and in appearance, being long and stout in Pentacrinus asterius and Pentacrinus 
miller: (Pls. XI., XIV.); shorter in Pentacrinus wyville-thomsoni, though still stout 
(Pl. XIX.); and slender in Pentacrinus naresianus, Pentacrinus decorus, and Penta- 
crinus blakei (Pls. XXVIUI., 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 Metacrinus costatus (Pl. XLIX.) and Metacrinus 
nodosus (Pl. L.), as in Metacrinus angulatus (Pl. XXXVUL), Metacrinus cingulatus 
(Pl. XL.), and Metacrinus murrayt (Pl. XLII). Tn the three types last mentioned, and 
also in Metacrinus interruptus (Pl. LII.), the cirri about the tenth or twelfth node are 
larger than those below them; but in other species this difference 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 wyvillit 
(Pl. XLVIIL), Metacrinus interruptus (Pl. LIL), and Metacrinus tuberosus (Pl. LIL) ; 
though it is less marked in Metacrinus varians (Pl. XLIV,). This character occurs in 
no Pentacrinus excepting Pentacrinus wyville-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 
