THE CRINOIDEA 



117 



passes dorsad of the axial canal, and the concentration of the 

 dorsal ligament in a pit at the end of the vertical ridge (Fig. 

 XXIII. 2). This is called " trifascial articulation." Further 

 increase of the ligamentar fossae and of the vertical ridge, 

 with the disappearance of the dorsal fossa, produces the 

 " bifascial articulation," adapted only for lateral movement (Fig. 

 XXIII. 3). These forms of articulation may be bilaterally 

 symmetrical, but in pinnuliferous brachials the fulcral ridge is 

 skewed, so that on the distal joint-surface the end of the ridge 

 towards the pinnule is moved dorsalwards. 



The syzygy (J. Miiller, 1841; P. H. Carpenter, 1884; 

 Bather, 1896) is an immovable sutural union between two brachials 

 of a pinnulate arm, accompanied with loss of the pinnule on the 



Fio. XXIII. 



Arm -joints. 1, brachial of Tsoeriwis asteria, after Job. Miiller. 2, distal face of IBr\ of 

 Tti'thy.rinns Aldrii-liiinnta (cf. 6 in Fig. XVIII.), x 8 diam. 3, the same in Isocrimis Blakei, x 3J 

 rtiani. 4, syzygy of Khizocrinus Rawsoni a, epizygal from its under surface ; h, hypozygal 

 from its upper surface, x TJdiam. 5, syzygy of Isocrinus Blakei a, upper surface of hypozygal ; 

 /', epizygal ; and c, hypozygal -in their relative positions, seen from side, x 3J diam. (2-5 are 

 after P. H. Carpenter.) 



in-, axial canal; ill, dorsal ligament fossa; il, interarticular ligament fossa; inf, muscle 

 fossa ; p', facet for pinnule ; rg, ventral groove. 



hypozygal (compare Fig. XVIII.). Immobility may be effected 

 in various ways. The apposed faces may be smooth (some 

 Pentacrinicls), striated (Uintacrinus, most Antedonidae), or dotted 

 (some Actinometrae) ; in Rhizocrinus a peg projects from the dorsal 

 region of the epizygal into a pit in the hypozygal (Fig. XXIII. 4), 

 and in some Pentacrinids a dorso-ventral ridge on the epizygal 

 fits into the hypozygal (Fig. XXIII. 5). The former type of 

 syzygy facilitates fracture along the suture, and is specially 

 developed in locomotive forms liable to entangle their arms. 

 The latter type appears different in origin and function. 



We now return to the extension of the Dorsal Cup. This may 

 be effected, as in Ichthyocriims (Fig. CVIIL), by the direct lateral 

 union of the proximal brachials. At the same time, the proximal 

 ambulacrals enter the tegmen, so that the thecal cavity stretches out 

 further between the actinal and abactinal elements. In many living 

 crinoids the proximal brachials are united by a flexible integument 



