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



Among the recent Pentacrinidse there is but one solitary example of a uniformly ten- 

 armed type, which embraces a majority of the species of Antedon. This is Pentacrinits 

 naresianus, represented in Pis. XXVIII.-XXX. 



Owing to the fragmentary condition in which many of the fossil Peutacrinidse occur, 

 it is impossible to say much about the nature of their arm- divisions. But in Perita- 

 crinus heaugrandi, de Loriol,' s])., the remains of the primary arms bear no axillaries up 

 to the twelfth joint 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 

 tubercidatus.^ 



Eeference has already been made to the low state of development of the arms of 

 recent Pentacrinidee as compared with those of Comatulas (ante, p. 55). They are fewer 

 in number (i.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 Comatulse. Singularly enough, 

 the two species Pentacrinus wi/ville-thomsoni and Pentacrinus aUernicirrus, in which the 

 distichal and palmar series are most uniform, are the very ones which most resemble the 

 Comatul^e in their mode of life {ante, p. 19). 



It is curious that in the Pentacrinidas and Apioerinidse the external appearance of the 

 arm-joints should be so much more constant than it is among the Comatulae. 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 pinnules which they bear. The tubercular projections on the pinnule-joints of 

 Pentacrinus asterius (PI. 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 Pentacnnus. But with the exceptions above mentioned the 

 pinnules of all the difi"erent species of Metacrinus are very much alike. Both in tliis 

 genus and in Pentacrinus the arm-joints are almost invariably of the transversely oblong 

 type (Pis. Xr., XIV. ; PL XV. figs. 2, 3 ; PL XVL ; PL XVIII. figs. 1^3 ; PL XIX. 

 figs. 1, 6, 7; Pis. XXV., XXXVL, XXXVIIL, XLIL, XLIIL-XLVL, XLVIIL, 

 XLIX., LIL); 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 



^ Monographie des Stages Jurassit^ues Supijrieurs de Bovilogne-sur-Mer, 2"" partie, p. 298, pi. xxxvi. fig. 23, a. 

 a Encriniden, Tab. 97, fig. 39. 



