188 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



narrow triangle, extends from a base coinciding with the inner side of the joint 

 face to an attenuated point at the outer distal angle. The greater part of the 

 outer half of the joint face beyond the central canal is occupied by an unpaired 

 interarticular ligament fossa. 



The articulation between the first and second segments of the distal pinnules: 

 This is essentially the same as the corresponding articulation in P,. 



The articulation between the sixth and seventh segment of the distal pinnules: 

 The dorsal border of the joint face is evenly convex; the ventral border is con- 

 cave; the two ventral angles are well rounded. There is the usual broad median 



fulcra! ridge. Occupying two pumpkin-seed 

 shaped areas, of which the rounded ends coincide 

 with the rounded ventrolateral angles of the joint 

 face and the sides converge inwardly to a point a 

 little more than half way to the median line and 

 slightly beyond the level of the central canal, are 

 two large and deep muscular fossre lodging a con- 

 spicuous pair of muscle bundles. 



Similar paired muscle bundles appear to oc- 

 cur at all of the articulations of the distal pin- 

 nules beyond that between the first two segments. 



ARTICULATIONS OF THE CIKRI. 



The articulation between the first cirrus seg- 

 ment and the centrodorsal is usually plane, but 

 the periphery of the joint face may be crenu- 

 late. or low ridges may extend inward for a short 

 distance toward the central canal. In the Atele- 

 crinidse transverse fulcra! ridges with deep fossae 

 on either side occur, which resemble those at the 

 articulations between the columnals in the larval 

 stem, and there are sometimes traces of trans- 

 verse ridges in other types. In its usual form 

 this articulation is syzygial, like that between the 

 centrodorsal and the topmost columnal. 



As noticed by W. B. Carpenter, each of the 

 cirrals in Antedon bifida is perforated by an axial 



canal (part 1, plate 13, fig. 587) of about one-fifth of its diameter, around which is a 

 raised articular surface. In the oral (ventral) and aboral (dorsal) sides of this pro- 

 jection there are two depressions lodging the connecting ligaments. In the basal seg- 

 ments (part 1, plate 13, fig 587, b) the canal with its surrounding raised area is cen- 

 tral, and the ventral and dorsal depressions are of equal size or nearly so ; but in the 

 terminal segments (part 1, plate 13, fig. 587, a) the canal and the articular surface are 

 nearer the ventral side and the ligament fossa is both larger and deeper on the dorsal. 

 The terminal claw is attached to the last segment by a plane surface so closely 

 that it is incapable of independent movement. 



FIG. 



230. LATERAL VIEW OF SPECIMEN 

 OF STROTOMETRA PRIAMUS. 



