REPORT ON THE CRINOIDEA. 



53 



two syzygies, and thus consists of three parts, and so do all the succeeding joints ; and 

 each joint gives off a pinnule from its distal end, the pinnules arising from either side of 

 the arm alternately." In this type, therefore, two-thirds of the arm-joints lose their 

 individuality altogether. They bear no pinnules and take no part in the movements of 

 the arms. In Rliizocrinxis half the bi-achials are in the same condition ; while more than 

 half are devoid of pinnules, as the lowest pinnule-bearing joint is the sixth or sometimes 

 even the eighth primitive brachial (PI. IX.). 



It is worth notice that the modes of arrangement of the arm-joints which are 

 characteristic of Hyocrinus and Rhizocrinus respectively, are precisely paralleled by the 

 condition of certain species of the Palaeozoic Heterocriniis. Thus in Heterocrinus con- 

 strictus, Hall, the pinnules are borne alternately on opposite sides of the arm by every 

 third joint ; and I have little doubt, from the figures of the arms which are given both by 

 HalP and by Meek,^ that each group of three joints is intersected by two syzygies just 

 as in Hyocrinus (PL VI. fig. 1). 



On the other hand, the alternation of syzygies and muscular joints, which is so 

 characteristic of Rhizocrinus, also occurs in Heterocrinus simplex ; and Meek's figures ^ 

 show that the opposed syzygial surfaces were striated as in Apiocrinus and Comatula, and 

 not plain as in Pentacrinus and Rhizocrinus. 



It has been pointed out already * that the suj^posed syzygies in the arms of Bathy- 

 crinus (PI. VII. fig. 2 ; PL VIII. figs. 1, 2 ; PL Villa, fig. 1) are really articulations of a 

 peculiar type, though the fossae and vertical ridge are barely visible in the outer parts of 

 the arms, and would probably have escaped notice altogether, but for the very marked 

 differences from ordinary syzygial surfaces which are presented by the apposed faces of 

 the two outer radials, or of two of the paired lower brachials (PL Vila. figs. 16, 19, 20, 22). 

 Nevertheless, the proximal joint of a pair so united resembles the hypozygal of a syzygy 

 in the non-development of its pinnule ; and it might therefore be urged that every pair 

 so united should be properly considered as a single joint, just as in the case of a syzygial 

 pair which only bears a pinnule on the epizygal. It must be remembered, however, that 

 the syzygial union is an immovable one, which is far from being the case with any arti- 

 culation, whether bifascial or trifascial ; and the reasons given above for retaining the 

 individuality of the two outer radials and of the first two joints beyond any axillary, even 

 when they are united by syzygy, apply equally well in the case of Bathycrinus. For the 

 hypozygal joints of syzygial pairs are not the only ones which never bear pinnules. The 

 lower joint of every pair forming a bifascial articulation is distinguished in the same way, 

 e.g., the first joints of the various arm-divisions in most Comatulse, and the first brachials 

 of Pentacrinus naresianus (PL XXXa. fig. 12&). The same is also true in the many- 

 armed Pentacrinidae, when there are many joints in an arm-division and the axillary is 



1 Twenty-fourth Annual Report on the New York State Museum of Natural History, Albany, 1372, pi. v. fiys. 13, 1-1. 



2 Palceontology of Ohio, vol. i. pi. i. fig. 10. ^ Ihid; pi. i. fig. 7. * Ante, pp. 8, 9. 



