38 THE VOYAGE OF H.M.S. CHALLENGER. 
The second brachials of Rhizocrinus (Pl. VIIIa. fig. 8, B;), and the second radials of 
Bathycrinus (Pl. Vib. fig. 6, A.) are in contact by their lower edges, but soon become 
entirely free from one another (Pl. VII.; Pl. VIII. figs. 1, 2; Pl. TX. figs. 1-3; Pl. X. 
figs. 1, 2, 6-8); while the corresponding plates in Hyocrinus (Pl. VI.) are absolutely 
free. But in many Crinoids the second radials are often very closely united by 
ligamentous bundles. These are lodged in fossee at the sides of the proximal face 
which is not quite so wide as the distal one (Pl. XII. figs. 9,10; Pl. XXI. figs. 5a, 5c). 
The first joints beyond every axillary are more or less closely united in the same way 
(Pl. XXI. figs. 3b, 4b). But the second joints and the axillaries themselves are free, ’ 
though often in very close contact with their neighbours, so that their apposed sides are 
more or less flattened (Pl. XXI. figs. 1b, 2b); and in some cases the first four or five 
of the free brachials have their sides flattened in this way, where they come in contact 
with one another and with the corresponding joints of adjacent rays (Pl. XV. fig. 2; 
PL XVI, fig. 1; Pl. XXXa. "fig. 8). 
In all the Pentacrinide there are invariably five rays. I have never met with any 
exception to this rule, and all the specimens of Bathycrinus that I have seen conform to 
it also. The original specimen of Holopus is tetramerous; while J have seen four 
Comatulee with the same peculiarity, and one with six rays. Four and six rays are more 
common in Rhizocrinus (Pl. VIIa. figs. 6, 7), and in very rare cases there may be seven. 
In the Comatulid genus Promachocrinus, however, ten is the normal number, but the 
basals are pentamerous. Only five rays extend outwards from the central rosette to 
appear externally beneath five of the radials (fig. 1, 4), and they must therefore be regarded 
as representing the primary interradii of the type. Hence those radial pieces which are 
not separated from the centro-dorsal by basal rays are the original embryonic radials, 
homologous with those of other Crinoids and of the five-rayed Star-fishes. The five 
others may perhaps be compared to the additional radials developed in many-armed 
Star-fishes, in which, however, the positions of the five primary rays are not indicated in 
the adult as they are in Promachocrinus. 
C. Tur INTERRADIALS. 
In almost all adult Neocrinoids the first radials meet one another all round the calyx 
so as to form a complete ring; and until lately this character has been regarded as one 
specially distinctive of the group. For in a large number of Palzeocrinoids an anal plate 
retains its primitive embryonic position and rests upon a basal, thus separating two of 
the radials and destroyimg the complete pentamerous symmetry of the calyx. A good 
instance of this type is the Carboniferous genus Belemnocrinus, which has a calyx very 
similar to that of the recent Rhizocrinus except for the presence of the intercalated anal 
