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



the stem immediately beneath the calyx, what appears to be the top stem-joint in a side 

 view having one or two more smaller joints resting in its upper surface but not reaching 

 the exterior as shown in PI. XXXIV. fig. 9. 



The top stem-joint — for the time being — of a specimen of Pentacrinus ivyville- 

 thomsoni is shown in PI. XXII. fig. 2. Resting on its upper si;rface is a smaller stellate 

 plate without any markings whatever, which in its turn would appear as the top stem- 

 joint until it was replaced by the development of another above it. As these joints are 

 relatively carried downwards from the calyx in succession by the appearance of younger 

 ones above them, they also become separated from one another by the intercalation of 

 new joints between them. Various stages of this process are shown in PI. XXII. 

 figs; 9-12, while fig. 4 shows an isolated young joint, and fig. 5 the depression in the 

 nest joint, which lodged it. Similar intercalated joints are shown in Pis. XXII. fig. 3, 

 and PL XXIII. figs. 1, 2. The result of this process is that the growing part of the stem 

 appears to consist of thick and thin joints alternating with one another (PI. XIII. fig. 1 ; 

 PI. XIV.; PL XV. figs. 1, 2; PL XVIII. fig. 1, 2; PL XIX. fig. 2, 6, 7 ; Pis. XXV., 

 XXVIII. -XXX.; XXXIV.-XXXVIL; PL XXXIX. fig. 1; PL XLIII. fig. 2; PL XLVIII. 

 fig. 2). The former are the older, the latter being subsequent additions. These 

 intercalated joints are always internodal, and the process goes on until the number of 

 joints between any two nodes reaches a certain average, which is constant for each 

 individual species. 



There are many indications that the increase in length of the stem of the fossil 

 Pentacrinidae was due to the same process. Quenstedt ^ gives an excellent figure of a 

 young concealed joint superposed upon an older and larger one, very much as shown in 

 PL XXII. figs. 9-12; and others of his figures upon the same plate illustrate the 

 difierent stages of growth on the stem of Extracrinus subangularis, as many as three or 

 four concealed joints being sometimes found intercalated between two of the larger ones. 

 In this species, too, with a stem which may reach 50 or 70 feet in length, the production 

 of nodal joints at the top of the stem must have been very rapid. According to 

 Quenstedt' forty or more succeed one another without any internodal joints being visible 

 externally, though the presence of concealed intercalated joints is revealed by the 

 examination of longitudinal sections of the stem. The final result of their growth was to 

 enormously increase the total number of internodal joints. 



Stem -fragments have been found by Quenstedt consisting of as many as eighty joints, 

 all internodal, and it is impossible to say how many more there may have been ; while 

 he states that he finds traces of small intercalated joints in almost every part of the stem. 



In most cases the new joints which have been intercalated between two older ones 

 eventually reach the same size as their predecessors, so that it is difficult to tell the 

 older from the younger joints in any mature stem. But in some species there appears to 

 1 Encriniden, p. 298, Tab. 101, figs. 24a, 246. ' Hid., p. 297, T;i1_). 101, figs. 16-19. 



