38 THE VOYAGE OF H.M.S. CHALLENGER'. 



The second brachials of Rhizocrinus (PI. Villa, fig. 8, B^, and the second radials of 

 Baihycrinus (PI. Vllb. fig. 6, R^ are in contact by their lower edges, but soon become 

 entirely free from one another (PI. VII.; PI. VIII. figs. 1, 2; PI. IX. figs. 1-3; PL X. 

 fio-s. 1, 2, 6-8) ; while the corresponding plates in Hyocrinus (PI. VI.) are absolutely 

 free. But in many Crinoids the second radials are often very closely united by 

 lio-amentous bundles. These are lodged in fossre at the sides of the proximal face 

 which is not quite so wide as the distal one (PI. XII. figs. 9, 10 ; PI. XXI. figs. 5a, 5c). 

 The first joints beyond every axillary are more or less closely united in the same way 

 (PI. XXI. figs. 36, 46). 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 (PI. XXI. figs, lb, 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 (PI. XV. fig. 2 ; 

 PI. XVI. fig. 1 ; PI. XXXa. fig. 8). 



In all the Pentacrinida3 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 I have seen four 

 Comatulse with the same peculiarity, and one with six rays. Four and six rays are more 

 common in Rhizocrinus (PI. Villa, figs. 6, 7), and in very rare cases there may be seven. 

 In the Comatulid genus Promachocrhms, 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 , a), 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 Promachoc7inus. 



C. The Intereadials. 



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 PaljBocrinoids an anal jjlate 

 retains its primitive embryonic position and rests upon a basal, thus separating two of 

 the radials and destroying 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 



