MOKOGRAPH OF THE EXISTING CBINOIDS. 185 



In manj' species of this group, as, for instance, Comanihus solaster, though the 

 young are as slender and as active as a young antedonid, the adults have the 

 proximal portion of their postradial series so closely united that it becomes diffi- 

 cult to trace the sutures and all possibility of intersegmental motion is lost. 



ARTICULATIONS OF THE PINNUI-ES. 



The articulations between the pinnulars and between the first pinnular and 

 the brachials have not as yet been studied in detail. 



Viewed from the exterior it is seen that the fulcral ridge of the articulation 

 between the brachial and the first pinnular (figs. 1083, 1085, 1090, 1095, 1096, pi. 18) 

 i.s roughly parallel to the transvei'se ridge on the adjacent brachial articulation; 

 the fulcral ridge of the articulation between the second and third pinnulars crosses 

 that of the articulation preceding at an angle of about 45°, while the fulcral ridges 

 of all the succeeding articulations run dorsoventrally (fig. 659, p. 329, and part 1, 

 fig. 54, p. 81). 



The two basal articulations show, in certain types, sufficient resemblance to 

 muscular articulations to remove all doubt in regard to their homologies, while in 

 otliers they are reduced almost to the status of the brachial synarthries. The 

 articulation between the second and third pinnulars and all following articulations 

 resemble the brachial synarthries, except that, in some if not most species, there is a 

 small muscle bundle just within each ventrolateral angle. 



Describing the articulations between the pinnulars in Antedon bifida W. B. 

 Carpenter wrote that the articular faces of the segments are formed on nearly the 

 same plan as those of the segments of the doreal cirri, the opening of the central 

 canal, by which everj' segment is traversed, being surrounded by a slightly elevated 

 ring, sometimes extended into a transverse ridge, and a depression being left by 

 the beveling away of the surface in both directions that serves for the lodgment of 

 interarticular ligaments. But besides these depressions there is in each of the 

 basal segments, at least of well-developed pinnules, a small but deep notch in the 

 ventral margin of each articular surface (fig. 1090, pi. 18), but deeper in the distal, 

 and this lodges a minute muscle, by the action of which the pinnules can be so 

 flexed that those of the two sides of the arm are brought toward each other, the 

 converse movement of extension being effected, as in the arms, by the elastic liga- 

 ments when the muscles are relaxed. 



P. H. Carpenter stated that each pinnule has a muscular attachment to the 

 brachial which bears it, while in the large tropical oligophreate species the lower 

 segments are united by muscles instead of by ligament or suture only. 



The pinnule articulations of H Imerometra martensi, based upon the study of a 

 large specimen from Singapore, may be described as follows : 



The articulation l)etween the second brachial and Pj : The joint face is some- 

 what broader than high and is shaped like the joint face on the distal end of 

 the radials. The fulcral ridge passes across the joint face approximately at 

 right angles to the longitudinal axis of the arm. The dorsal ligament fossa is 

 deeply excavated and is moi-e broadly rounded inwardly than outwardly ; a promi- 



