26 THE VOYAGE OF H.M.S. CHALLENGER. 



But I should prefer doing this to using a name, as de Loriol has done, which is so very 

 similar to that universally employed to designate some of the calyx plates, although 

 there is no sort of homology between the two structures. This latter point is recognised 

 by de Loriol, who makes it clear that he regards the " article basal " as a stem-joint. 



Whatever may be the case in Apiocrinus, this plate is single in Rhizocrinus rawsoni 

 (PI. X. fig. 9), and I cannot help suspecting that Zittel has been misled by the appearance 

 of sutures into regarding it as probably consisting of five coalesced under-basals. I 

 have noted a somewhat similar condition in Millericrinus pratti} 



Owing to the larger number of discoidal joints in the stem of Rhizocrinus rawsoni 

 than in that of Rhizocrinus lofotensis, the gradual development of their articular faces 

 is more easily made out in the former species. As the joints become longer than wide, 

 shallow fossaj appear to the right and left of the opening of the central canal, which 

 thus seems to be the deep middle portion of an oval depression occupying the shorter 

 axis of the elliptical face (PI. X. fig. 12). These fossae gradually increase in relative 

 size, and encroach more and more upon the original plane surface of the joint face, 

 stm, however, remaining in connection with one another around the central canal 

 (PI. X. figs. 11, 13, 14). They reach nearly to the margin of the joint face, so as 

 to leave a small articular rim outside them ; but they do not reach so far in the direction 

 of the long axis, at the ends of which the original surface of the joint remains to form 

 the so-called " articular ridge." This is, however, by no means continuous across the 

 opening of the central canal, as is implied by its name ; for it is merely represented by 

 two triangular surfaces which encroach upon the original oval depression so that it 

 assumes the form of a i-ather short-handled dumb-bell (PL X. figs. 11, 13, 14). The 

 two ends slope rapidly downwards towards the centre, where the opening of the axial 

 canal is situated. It thus establishes a connection between the two fossae and interrupts 

 the continuity of the articular ridge. 



This ty[5e of joint-face also occurs in the lowest part of the stem of Bathycrinus 

 (PI. Vila. figs. 10, 11), but it is the result of a different mode of growth altogether. 

 There are several thin discoidal joints at the top of the stem (PI. VII. figs. 1-3, 11 ; 

 PL Villa, fig. 1), and the uppermost one, on which the basals rest, has its surface 

 marked by a ten-rayed depression which extends outwards from the five-lobed opening 

 of the central canal (PL Vila. fig. 3). The corresponding face of the basal ring is 

 marked in the same manner (figs. 13, 14); and the fossae lodge the five horse-shoe 

 shaped bgamentous bundles which unite the basals to the stem-joints below them. 

 On the upper face of the second joint, however, the opening of the central canal 

 is surrounded by a raised articular rim, stiU showing traces of fossse like those on the 

 top joint, and this rim is more marked on the next few joints (figs. 4, 5). As the 

 joints below become thicker and their terminal faces more oval, the articular rim also 



1 Ou some new or little known Jurassic CrinoiJs, Quart. Journ. Gcol. Soc, vol. xxsviii. p. 34. 



