146 THE VOYAGE OF H.M.S. CHALLENGEE. 



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 Vila. fig. 15 ; PL Vllb. figs. 1, 5 ; 

 PL Villa, fig. 7 ; PL X. figs. 1-4 ; PL XII. figs. 9, 12, 15, 20, 23 ; PL XX. figs. 7, 8 ; 

 PL XXI. figs. 5c, 6d ; PL LXIL). It presents itself even in forms which have such decided 

 Palaeozoic afiinities as Hyocrinus, Marsupites, and Thaumatocrinus, but it appears, accord- 

 ing to de Loriol,^ to be absent in Guettardicrinus, in which type the reduced articular 

 facet found in Ajyiocrinus seems to have disappeared altogether, not only from the distal 

 faces of the first radials, but also from those of the axdlaries. 



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 

 weU-marked articular facets as those of the Pentacrinidge, 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 Pentacrinida^ and Comatulse. Marsiqntcs, 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, Foteriocrinus, 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 joint 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 fossse for muscles and ligaments, 

 the central canal, and the transverse articular ridge of the radials and arm-joints of aU 

 the post-Palseozoic Crinoids (except Guettardicrinus) 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 Paleont. Frang., oj). cit., pp. 216, 218. ' Op. cit., p. 221. 



^ See Wachsmuth, Amer. Journ. Sci. and Arts, vol. xiv. p. 190. 



