MONOGRAPH OF THE EXISTING CRINOIDS. 165 



In the genera Thalassocrinus (part 1, fig. 145, p. 209) and Gephyrocrinus of the 

 Plicatocrinidse there are about as many nonmuscular as muscular articulations, but 

 in the genus Hyocrinus there are two nonmuscular articulation to one muscular. 



As exceptions to this rule we find the genera Phrynocrinus (part 1, fig. 2, 

 p. 61), Ptilocrinus (part 1, fig. 144, p. 207), and Calamocrinus, in the arms of which 

 nonmuscular articulations are relatively infrequent, and Proiscrinus (part 1, fig. 

 128, p. 199), in which there are none beyond the synarthry in the first brachial pair. 



Scattered at more or less regular intervals throughout the comatulid arm are 

 pairs of brachials very closely united by an articulation consisting of ligament fibers 

 only. The apposed faces of the brachials so united show, running to a more or less 

 broad raised rim about the central canal, prominent radiating ridges of unusually 

 dense and compact calcareous structure with furrows between them, which on the 

 two adjoining articular faces correspond in position (figs. 1089, 1096, pi. 18, and 

 part 1, fig. 34, p. 71). Externally two brachials so imited appear as a single 

 brachial, usually slightly longer than those on either side, crossed by a fine white 

 line at right angles to the longitudinal axis of the arm (fig. 1092, pi. 18), which 

 imder magnification appears dotted, the dots representing the ends of the little 

 blind canals beetween the apposed ridges of the two joint surfaces. The ligaments 

 binding together two brachials united in this way are identical in character with 

 the ligaments in the muscular articulations, in the synarthries, in the articulations 

 between the pinnulars and cirrals, and between the radials, but are much shorter. 

 The fibers are very numerous and closely packed along the apposed ridges, but more 

 scattered in the furrows between them. 



Such an articulation is laiown as a syzygy (part 1, figs. 6, p. 63, 30, p. 71), and 

 two brachials united by syzygy form a syzygial pair (figs. 1090-1092. pi. 18). 



The number of radiating ridges on the apposed faces of a syzygial articulation 

 varies from 5 in Atopocrinus sihogo! (part 1, fig. 227, p. 245) to at least 50 in 

 Amphhnetra discoidea, but is usually between 15 and 40. It appears to be to a 

 certain extent proportionate to the size of the articular face, and is greater in the 

 Oligophreata than in the Macrophreata. 



Generally speaking the ridges are straight, and run from the periphery of the 

 joint face toward the central canal ; many of them are short, running for a greater 

 or lesser distance inward and tapering to a slender point which may lie anjrwhere 

 between the periphery and the central canal, while others coalesce with a neighbor- 

 ing ridge, the two continuing as a single ridge to the canal. 



It not infrequently happens in the outer part of the arms that an especially 

 large and broad ridge runs in the median line from the canal to the middle of the 

 dorsal border of the joint face, or two or three noticeably straight, strong, and 

 prominent ridges may be developed here. AVhen this occurs the remaining ridges 

 are commonly more or less undeveloped, terminating at some distance from the rim 

 about the central canal, so that we see an approach to the sj^narthrial type of 

 articulation. 



