200 THE VOYAGE OF H.M.S. CHALLENGEE. 



face on the dorsal side of the pit (PI. III. figs. 14, 15). But on the lower arm-joints 

 and on the radials there is practically nothing of this kind, and the pit lies immediatel}' 

 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 PL III. figs. 5-13. 

 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 theii" 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 Hnes of the 

 transverse articidar 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 fossae for the attachment of the muscles and the interarticular lio-aments 

 is remarkably difi"erent 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 O'OIS to 0"035 mm. in diameter, and disposed in regular rows with then- long- 

 axes tangential. Here and there, as shown in PL 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 w-eU 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 ]\Ir. 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 \. 

 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 ficr. 3. 



O 



The central funnel is here much narrowed, and the lines of the ai-ticular ridges are 

 seen at a distance of about 1 mm. from its opening. Outside the pentagonal figure 



