No. 514] ABTICULATIOXS OF CIUNOIDS 



579 



bound together with ligament fibers similar to those of 

 the dorsal ligament of muscular articulations; in syn- 

 arthries (Fig. 5) these fibers are segregated into two 

 large bundles lateral in position, separated by a dorso- 

 ventral ridge across the joint face; in syzygies (Fig. 11) 

 the fibers are scattered over the whole joint surface, 

 which is broken up into alternating ridges and furrows 

 radiating outward from the central canal, which may, in 

 certain of the more specialized forms, such as the Pen- 

 tacrinitidae, become obsolete except about the periphery 

 of the joint face. 



Muscular articulations are often doubled, thus forming 

 an axillary from which two arms arise; this never hap 

 pens in the case of non-muscular articulations ; moreover, 

 muscular articulations are primarily pinnulate, the pin- 

 nule arising from a pinnule socket in the proximal outer 

 part of one of the muscular fossae. The difference between 

 straight and oblique muscular articulations was orig- 

 inally a difference in pinnulation. In the most primitive 

 type of crinoid arm found among the recent forms, oc- 

 curring in the family Pentametrocrinida?, we find the fol- 

 lowing sequence of articulations: (1) straight muscular, 

 uniting the radial to the first post-radial joint, (2) syn- 

 arthry, uniting the first two post-radial joints, (3) oblique 

 muscular, uniting the second and third post-radial joints; 

 all the succeeding articulations are oblique muscular, 

 except for the interpolation of occasional syzygies. The 

 first oblique muscular articulation bears the first pinnule; 

 the addition of a pinnule socket on one side of the joint 

 face causes a certain amount of crowding, and a conse- 



