226 



THE VOYAGE OF H.M.S. CHALLENGER. 



developed from the thin disks that successively appear immediately beneath the calyx 

 are different in the two genera {ante, pp. 26, 27). There is alwaj^s a large number of 

 these thin joints at the top of the stem of JBathycrinus (PI. VII. figs. 1-3, 11; PI. Villa, 

 fig. 1), whereas in Rhizocrinus (PL IX. figs. 1-3 ; PL LIII. figs. 7, 8) there are very 

 few, often not more than one or two, and these by no means so thin as in Bathycrinus. 



An entire stem, or the upper and middle part of one, could therefore be referred 

 without difiiculty to its proper genus. But the lower and middle joints are so much 

 alike in the two genera that the pi'oper identification of a fragment or of isolated joints, 

 either recent or fossil, would become a matter of uncertainty, if not of impossibility. 



The genus Bathycrinus was never formally defined by Sir Wyville Thomson ; but in 

 his first account of it ^ he said that, like Rhizocrinus, it " must also be referred to the 

 Apiocriuidse, since the lower portion of the head consists of a gradually exjDanding funnel- 

 shaped piece, which seems to be composed of coalesced upper stem-joints;" and he 

 nowhere mentioned the presence of any calycular plates below the radials. Subsequently, 

 however, he stated,^ after examining Bathycrinus aldrichianus, that the stem of this 

 genus " barely enlarges at its junction with the cup ; " and he described the lower portion 

 of the latter as consisting of a series of basals which are soldered together into a small 

 ring, scarcely to be distinguished from the upper stem-joint (PL VII. figs. 1, 2, 11 ; 

 PL Vila. figs. 12-14 ; PL Vllb. figs. 1, 2). 



The existence of basals in Ilycriniis {Bathycrinus) car- 

 penteri was also recognised by Danielssen and Koren,^ who 

 were fortunately able to see the interbasal sutures in young 

 individuals, though these entirely disappear in the adult. 



Although invisible on the upper and lower surfaces of the 

 basal ring of Bathycrinus aldrichianus, as well as externally 

 (PL Vila. figs. 12-14), the sutures are clearly seen in sections 

 through its middle portion (PL Vllb. fig. 2). It expands very 

 slightly from below upwards, and its somewhat hollowed under 

 surface is marked by ten fossje radiating outwards from the 

 centre and separated by intervening ridges (PL Vila. fig. 14). 

 They correspond to similar fossae on the upper face of the thin 

 top stem-joint (PL Vila. fig. 3), and lodge five strong but short 

 interradial ligamentous bundles, each having somewhat the 

 form of a horseshoe or V with thick limbs (woodcut, fig. 11, bl). 

 These, as already described, unite the basals to the thin, u^aper 

 stem-joints, and are gradually replaced as the joints become thicker by the two larger bundles 

 which form cushion-like pads between every two of them (ante, p, 27; PL Vila. figs. 4-6). 



Fig. 11. — Diagram of a horizontal 

 sectiou through the lowest portion 

 of the basal ring of Bathycrinus 

 aldrichianus ; x 70. bl, ligaments 

 uniting the basals to the top 

 stem-joint ; ch', the outer vessels 

 in the vascular axis, wliich are 

 continued downwards from the 

 chambers of the chambered organ ; 

 if, interradial portions of the fib- 

 rillar sheath round the vascular 

 axis which are separated by rs, 

 the radial spaces in the upper part 

 of the stem ; v, central vessel of 

 the vascular axis. 



1 The Depths of the Sea, p. 450. 



3 Nyt Mag. f. Naturvidensk., Bd. xxiii. pp. 4, 5. 



2 Journ. Linn. Soc. Land. (ZooL), vol. xiii. (1876) 1878, x^p. 48, 50. 



