200 THE VOYAGE OF H.M.S. CHALLENGER. 
face on the dorsal side of the pit (Pl. III. figs. 14, 15). But on the lower arm-joints 
and on the radials there is practically nothing of this kind, and the pit hes immediately 
next to the dorsal edge of the articular face. But the dorsal surface of the joint is 
strongly convex and produced far below this edge, as is well shown in PI. III. figs. 5-18. 
This is also the case with the distal portions of the first radials, as may be learnt from a 
comparison of figs 1 and 2 on Pl. V. The latter represents a horizontal section of the 
radials which passes 2 mm. below their edge on the concave (bivial) side, and 7°5 mm. 
below it on the convex (trivial) side. 
Around the opening of the central funnel, which is narrower than at the top of the 
calyx, is an irregularly shaped pentagonal figure. This is formed by the lines of the 
transverse articular ridges, which in Holopus, as in all other Crinoids, are formed 
of a much closer and denser limestone reticulation than the remainder of the skeleton. 
Immediately within these lines are the indications of the small openings of the central 
canals of the radials; and just outside them are the ends of the pits lodging the dorsal 
ligaments. The texture of the limestone network forming the inner faces of the radials 
and the fossee for the attachment of the muscles and the interarticular ligaments 
is remarkably different from that of the outer portion of the cup. The two are separated 
by the lines of the transverse articular ridge, as is shown in fig. 2 on Pl. V. and more 
distinctly in fig. 7, where the dark line indicates the position of the articular ridge. The 
substance of the radials inside this line is formed of an irregularly open network, the 
meshes of which reach 0°08 or 0°09 mm. in diameter, though many of them are much 
less, sometimes not a quarter that width. 
The peripheral portion of the cup, however, is formed of a much more regular net- 
work. ‘This consists of concentric and radiating rods which enclose circular or elliptical 
meshes from 0°015 to 0°035 mm. in diameter, and disposed im regular rows with their long 
axes tangential. Here and there, as shown in PI. V. fig. 7, the lines of the meshwork 
are a little irregular, but its general character is very uniform. The difference between 
the two types of network is most marked, as much in the regularity as in the size of the 
meshes, as is well shown in the inner and outer portions of fig. 7, and also in the 
enlarged portions of limited areas which are represented in figs. 5 and 6. Fig. 8, on the 
same plate, is an ideal diagram, constructed by Mr. Black, showing the regular disposi- 
tion of this peripheral reticulation. The difference of the two textures is obvious enough 
to the naked eye; but it becomes more apparent with the help of a lens which brings out 
the regularly striated aspect of the outer part of the cup. This is well shown in Pl. VY. 
fig. 2, and less clearly in fig. 4, which represents a section taken about 5 mm. above the 
basal expansion, and corresponding to the upper face of the vertical section shown 
in fig. 3. 
The central funnel is here much narrowed, and the lines of the articular ridges are 
seen at a distance of about 1 mm. from its opening. Outside the pentagonal figure 
