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 always a large number of 
these thin joints at the top of the stem of Bathycrinus (Pl. VI. figs. 1-3, 11; Pl. VIIa. 
fig. 1), whereas in Rhizocrinus (Pl. IX. figs. 1-3; Pl. LUI. 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 difficulty to its proper genus. But the lower and middle joints are so much 
alike in the two genera that the proper 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 
Apiocrinide, since the lower portion of the head consists of a gradually expanding 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 
2 
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-jomt (Pl. VII. figs. 1, 2, 11; 
Pl, Vila. figs. 12-14; Pl. VIIb. figs. 1, 2), 
The existence of basals in Ilycrinus (Bathycrinus) car- 
pentert 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 (PI. VIIb. fig. 2). It expands very 
Fie. 11.—Diagram of a horizontal 
section 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, which are 
continued downwards from the 
chambers of the chambered organ ; 
af, interradial portions of the fib- 
rillar sheath round the vascular 
axis which are separated by 7s, 
the radial spaces in the upper part 
of the stem; v, central vessel of 
the vascular axis. 
slightly from below upwards, and its somewhat hollowed under 
surface is marked by ten fosse radiating outwards from the 
centre and separated by intervening ridges (Pl. VIIa. fig. 14). 
They correspond to similar fossee on the upper face of the thin 
top stem-joint (Pl. VIIa. 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, b/). 
These, as already described, unite the basals to the thin, upper 
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. VIIa. figs. 4-6). 
1 The Depths of the Sea, p. 450. 
2 Journ. Linn. Soc. Lond. (Zool.), vol. xiii. (1876) 1878, pp. 48, 50. 
3 Nyt Mag. f. Naturvidensk., Bd. xxiii. pp. 4, 5. 
