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



following joints being axillaries. This is in accordance with the nomenclature employed 

 by Zittel, who speaks of the joint in Cui^ressocrinus, which is caUed " articulare " by 

 Schultze, and "second radial" by Eoemer, as a "first brachial ;" while he onlj' describes one 

 series of radials in the five-armed Pisocrinus.^ The developmental history of the plates 

 also indicates clearly that the second and following radials are really ann-joints. For 

 they commence as imperfect rings, which soon become filled up with lengthening 

 fasciculated tissue, just as is the case with the stem-joints and later brachials. But the 

 first radials, like the basals and orals, commence as expanded cribriform films ; while the 

 endogenous additions by which they are subsequently thickened are cribriform like those 

 of the basals, and not fasciculated like those of the two outer radials and the following 

 arm-joints. Messrs. Wachsmuth and Springer' have been led by their study of the 

 Palseocrinoids to the same conclusion, i.e., that " the arms fundamentally commence with 

 the second radials ;" although they find in practice that for purposes of description " it is 

 more convenient to regard the arms as commencing with the first free plate beyond the 

 calyx." In very many Neocrinoids with ten or more arms this would be the second 

 radial ; and in the multiradiate Metacrinus (PL XXXVIIL; PI. XLIII. fig. 2 ; PI. XLVL; 

 Pis. XLVIII.-LII.) this is actually a syzygial joint with a pinnule on the epizygal just 

 as in the simpler Eudiocrinus indivisus, but an axillary appears a few joints farther on, 

 and the rays begin to divide. In the other Pentacrinidse, however, in Bathycrinus, 

 Iloloims, and in most Comatulse, as well as in the fossil Encrimis and Apiocrinidse, the 

 second joints above the primary radials are axillaries, and it is not tiU the second (or 

 rarely the first) joints beyond these that pinnules appear. In all these t}^es the axillary 

 and the joint immediately below it are of the same width as the primary radials in the 

 calyx. But in Marsupites and in many Palseocrinoids [Platycrinus, Cyathocriniis, &c.) 

 they are very much smaller than the primary radials, just as the homologous joints are 

 in Hyocrinus (PL VI.). 



The primary radials which form the upper part of the calyx are generally distin- 

 guished as i)\Q first radials ; while the following joints, as far as the first axillary inclusive, 

 are called the second, third radials, &c., though they are really only arm -joints as is shown 

 by their bearing pinnules in Metacrinus (PI. XII. figs. 6, 8 ; PL XXXVIIL ; PL XXXIX. 

 fig. 1 ; PL XLIII. fig. 2 ; PL XLV. fig. 1 ; PL XLVL ; PL XLVIII. fig. 1 ; PL XLIX. 

 figs. 1, 2 ; PL L. figs. 1, 8, 10, 14, 16 ; PL LI. fig. 1 ; PL LII. fig. 1). Since, too, it is 

 very convenient for descriptive purposes to use different names for the diflferent regions 

 of the arms, I see no reason for altering the names by which these plates are generally 

 known, provided that their real nature is not lost sight of. 



The conventional use of the term " radials " for the joints between the calyx and the 



1 PalcBontologie, pp. 348, 349. 



2 Phil. Tram., 1865, p. 541, pi. xxvii. figs. 1, 3 ; Ibid., 1866, pp. 729, 742, pi. sli. fig. 1. 



3 Kevision, part ii. p. 10. 



