146 THE VOYAGE OF H.M.S. CHALLENGER. 
reduced to a supposed difference in the mode of union of the first radials with the joints 
which they bear. 
In all the Mesozoic and later Crinoids this is an articulation effected by a pair of 
muscles and three ligaments, as described above (Pl. VIIa. fig. 15; Pl. VIIb. figs. 1, 5; 
PLeV Ilia. ifie. 7; Pl. X. fies. 14; Pl xa vies. 9, 12, 15, 20.23; WORX. has se 
Pl. XXL. figs. 5c, 6d; Pl. LXIL). It presents itself even in forms which have such decided 
Palseozoic affinities as Hyocrinus, Marsupites, and Thawmatocrinus, but it appears, accord- 
ing to de Loriol,’ to be absent in Guettardicrinus, in which type the reduced articular 
facet found in Apiocrinus seems to have disappeared altogether, not only from the distal _ 
faces of the first radials, but also from those of the axillaries. 
In this respect, therefore, as also in the presence of numerous interradials, and in the 
close lateral union of the lower parts of the rays, Guettardicrinus is a true Tessellate, as 
has been already pointed out by Liitken.? 
On the other hand, the first radials of many of the so-called Tessellates have just as 
well-marked articular facets as those of the Pentacrinide, and the joints belonging to the 
other orders of radials are united to one another in just the same way as the distichals 
and palmars of Pentacrinidee and Comatule. Marsupites, Allagecrinus, and Platycrinus 
are notable examples of the presence of a muscular joint between the first and second 
radials; and I feel confident that it would be found to recur in numerous other forms 
with perforated first radials, such as Cyathocrinus, Poteriocrinus, and Myrtillocrinus, if 
properly looked for. 
Thus then, so far as regards the presence or absence of a true articulation between 
the first and second radials, no hard and fast line can be drawn between the older and 
the younger Crinoids: Many of the former certainly had a muscular jomt in this 
position; while it was absent in Guettardicrinus, though not from an arrest of 
development, as in the older Crinoids. ‘ 
The want of a distinct articular facet on the first radials of many Tessellata is only 
another indication of their being in the condition of permanent larval forms, as is so 
clearly shown by many of their other characters.’ The fossze for muscles and ligaments, 
the central canal, and the transverse articular ridge of the radials and arm-jomts of all 
the post-Paleozoic Crinoids (except G'uettardicrinus) only make their appearance at a 
comparatively late developmental stage. The axial cords of the young joints, whether of 
rays, arms, or pinnules, lie at first upon their upper surfaces, eventually being received 
more deeply into the substance of the plates. They lie for some time in open canals, 
which are only closed up and received into the substance of the skeleton at a later 
period. Consequently the transverse section, which at first resembles a horse-shoe, 
finally comes to be a ring, with muscles and ligaments attached round it. Now in many 
1 Paléont. Frang., op. cit., pp. 216, 218. 2 Op. cit., p. 221. 
3 See Wachsmuth, Amer. Journ. Sci. and Arts, vol. xiv. p. 190. 
