734 MR. p. H. CARPENTER ON THE [DcC. 19, 



joints which become immovably united and behave in all respects as 

 one. But it mav be united by syzygy to the second radial, instead 

 of by the usual bifascial articulation^ This, however, is of an alto- 

 gether different morphological value from the syzygial unions of the 

 arm-joints. In the latter case the hypozygal entirely loses its indi- 

 viduality as a separate joint, and bears no pinnule as the epizygal 

 and the remaining brachials do. Thus, for example, in very nearly 

 all (JomatulcB the original third and fourth joints of the growing arm 

 differ from those which ultimately appear beyond them. For 

 " whilst the majority of these gradually come to possess the true 

 articulations, and to be separated by the intervention of muscles and 

 ligaments, a certain small proportion become more intimately united 

 on a simpler plan, which admits of no motion between them"-. The 

 double or syzygial joints thus formed resemble the ordinary brachials 

 in bearing but one pinnule, and they are therefore best considered as 

 single joints. In Ant. rosacea, for example, the 3rd and 4th, the 9th 

 and 10th, and the 14th and 15th joints of the growing arm are 

 respectively united in pairs by syzygy; but the arm is best described 

 as having syzygies in the 3rd, 8th, and 12th joints. So again in the 

 numerous ComatuIcBy such as Act. parvicirra, which have axillaries 

 on some or all of the primary arms. Counting from the third radial, 

 the distichal axillary is primitively the fourth joint. The first, as is 

 almost invariably the case, bears no pinnule, while the second does 

 bear a pinnule, but the third not ; for it is united to the following 

 (axillary) joint by a syzygy. The first ray-division would therefore 

 be described as consisting of three distichal joints, the second bearing 

 a pinnule, and the third (axillary) being a syzygy. 



In Encrinus, in most recent species of Pentacrinus, in a few Coma- 

 tulae {Act. Solaris, &c.) the two outer radials and the first two joints 

 beyond them are respectively united by syzygy ; and on the principle 

 explained above, each pair would be considered as forming a single 

 joint, so that the true third brachial (itself a syzygial joint) would 

 come to be the second. This would involve our describing these 

 forms as having but two radials, the axillary with a syzygy, and 

 syzygies both in the first and in the second brachials. I think, 

 however, that this would be misleading and make the difference be- 

 tween the two types appear much greater than it really is. 



The presence of three radials is such an absolutely constant 

 character in all the five-rayed Neocrinoids, excepting Metacriiius & 

 P/icatocrinus, that the fact of the two outer ones being united by 

 syzygy and not articulated seems to me to be of minor importance ; 

 and I do not assign to it the same morphological value as the 

 syzygial union of the third and fourth primitive brachials, in which 



^ I use this name to denote the presence of a ligamentous bundle on either 

 side of a vertical articular ridge, while muscles are absent. In by far the greater 

 number of Coviatula this is the mode of union of the two outer radials and 

 of the first two joints beyond each axillary. It also occurs in MiUericriiius, 

 Apiocrinus, Fenfacrhiirs decorus, P. blakei and other types. But in Bathycrinus 

 there is a trifascial articulation, a small additional bundle of fibres being inserted 

 into the apposed faces of the joints at the lower or dorsal end of the articular 

 ridge. 



- W. B. Carpenter, Phil. Trans. 1866, p. 721. 



