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 Pl. XXXIV. fig. 9. 
The top stem-joint—for the time being—of a specimen of Pentacrinus wyville- 
thomsoni is shown in Pl. XXII. fig. 2. Resting on its upper surface 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 Pl. XXII. 
figs. 9-12, while fig. 4 shows an isolated young joint, and fig. 5 the depression in the 
next joint, which lodged it. Similar intercalated joints are shown in Pls. XXIL. 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 (Pl. XIII. fig. 1; 
PIX. Ply MV. figs. 1, 25, Pl.) XV fig. 1, 25) Pl KEK hie, 2G feels SAY. 
XV XK KXKIV.-XXKVIL;, Pl. XXXIX. fig, 1; (PL SEM hs. 27 PX Eye 
fig. 2). The former are the older, the latter bemg subsequent additions. These 
intercalated joimts 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 
Pentacrinidze 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 
different stages of growth on the stem of Hatracrinus 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 jomts 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 jomts 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, 240. 2 Thid., p. 297, Tab. 101, figs. 16-19, 
