286 THE VOYAGE OF H.M.S. CHALLENGER. 
Among the recent Pentacrinide there is but one solitary example of a uniformly ten- 
armed type, which embraces a majority of the species of Antedon. This is Pentacrinus 
naresianus, represented in Pls. XX VIII.—XXX. 
Owing to the fragmentary condition in which many of the fossil Pentacrinidee occur, 
it is impossible to say much about the nature of their arm-divisions. But in Penta- 
crinus beaugrandi, de Loriol,’ sp., the remains of the primary arms bear no axillaries up 
to the twelfth jomt from the radials; while eleven simple joints are still preserved in the 
specimen from the Lias of Vaihingen, which is referred by Quenstedt to Pentacrinus 
tuberculatus.” 
Reference has already been made to the low state of development of the arms of 
recent Pentacrinide as compared with those of Comatulee (ante, p. 55). They are fewer 
in number (7.e., when multiradiate forms are compared), and have both pinnules and 
ambulacral plating less developed towards their ends; while the number of joints 
separating successive axillaries is far more variable within specific limits, and does not 
seem to have become tolerably fixed as is the case in the Comatule. Singularly enough, 
the two species Pentacrinus wyville-thomsoni and Pentacrinus alternicirrus, in which the 
distichal and palmar series are most uniform, are the very ones which most resemble the 
Comatulee in their mode of life (ante, p. 19). 
It is curious that in the Pentacrinidee and Apiocrinidee the external appearance of the 
arm-joints should be so much more constant than it is among the Comatule. In the 
latter family the arm-joints may be saucer-shaped, more or less sharply wedge-shaped, 
&c., and it is in many cases easy to identify a species from detached portions of the arms, 
especially as there is also very considerable variation in the characters of the pinnules. 
But both in Pentacrinus and in Metacrinus there is a very great sameness, not only in 
the form of the arm-joints as seen from their dorsal side, but also in the appearance of 
the pimnules which they bear. The tubercular projections on the pinnule-joints of 
Pentacrinus asterius (Pl. XIII. figs. 1, 14), and the indications of carination on the 
pinnules of a few species of Metacrinus, are almost the only variations in the character of 
the pinnules through all the recent species of the family. It is true that the features of 
the lower pinnules of Metacrinus are such as to afford a character of some generic value 
for separating it from Pentacrinus. But with the exceptions above mentioned the 
pinnules of all the different species of Metacrinus are very much alike. Both in this 
genus and in Pentacrinus the arm-joints are almost invariably of the transversely oblong 
type (Pls: XT, XIV.; PL XV, figs. 2, 3; Pl: XVI; gee eile fies: 13 exes 
figs. 1, 6, 75 °Pls. XXV., XXXVI, XXXVING) DER eX EIEN va ei vais 
XLIX., LII.); and the same is the case in most, if not all, of the fossil species. It is 
therefore at first sight by no means easy to identify the species to which isolated 
1 Monographie des Etages Jurassiques Supérieurs de Boulogne-sur-Mer, 2™° partie, p. 298, pl. xxxvi. fig, 23, a. 
2 Encriniden, Tab. 97, fig. 39. 
