THE 



AMERICAN NATURALIST 



Vol. XLIII October, 1909 No. 51 I 



THE NON-MUSCULAR A RTICULATK )XS OF 

 CRINOIDS 



AUSTIN HOBART CLARE 



It has been long known that the non-musenlar articula- 

 tions in the crinoid arm, synarthries or bifascial articula- 

 tions, and syzygies, have an entirely different effect 

 upon the arm structure than do articulations possessing 

 muscle bundles, straight or oblique muscular articulations. 

 The muscular articulations are composed of three ele- 

 ments (Figs. 1 and 7) ; (1) the dorsal ligament, bounded 

 ventrally by a strong transverse ridge running across the 

 middle of the joint face, (2) the interarticular ligaments, 

 just ventral to the transverse ridge, occupying triangular 

 areas one on each side of the central canal, and (3) the 

 muscle bundles, occupying two large distally rounded 

 areas, separated by a narrow median ridge or furrow; in 

 straight muscular articulations (Fig. 1) the transverse 

 ridge separating the dorsal ligament fossa from the inter- 

 articular ligament fossa 1 runs at right angles to the dorso- 

 ventral axis of the joint face, and the two interarticular 

 ligament fossae and two muscular fossa? are of equal size, 

 while in oblique muscular articulations (Fig. 7) the trans- 

 verse ridge is strongly diagonal in position, and the two 

 interarticular ligaments and two muscular fossae are, on 

 one side crowded, on the other drawn out, and therefore 

 unequal. 



The non-muscular articulations (Figs. 5 and 11) are 



