I J BULLETIN 82, 1 N'lTED STATES NATIONAL MUSEUM 



inner median, and vice versa. It reaches such a position that its muscular fossae still 

 continue adjacent to the muscle pit of the pinnular side of the articular face of the 

 brachial. The dorsal ligamenl of the pinnule facet, on the other hand, seems to be 

 removed almost to the maximum distance from the dorsal ligament of the brachial 

 facet, if the brachial be viewed in plane projection. The pinnule facet forms an 

 angle of about 110° with the articular face of the brachial, in, for instance, Helio- 

 iit, tin and Himerometra. At the distal end of the IBr 2 in Metacrinus nobilis tenuis 

 the corresponding angle was 140°. The dorsal ligament of the pinnule facet is some 

 times better developed on the side adjacent to the median part of the brachial, and 

 narrower on the side facing the lateral portion of the brachial. On the ventral side 

 of the transverse ridge the lateral portion, on the contrary, is always more strongly 

 developed. The intermuscular crest or furrow therefore bends considerably toward 

 the median part of the brachial. The proximal part of the first pinnular answers to 

 the pinnular joint face described on the brachial. I lere, however, the articular fossae 

 an- often indistinctly marked, and the joint face only bulges irregularly, especially in 

 small forms. When the muscles in this articulation are contracted the pinnule is 

 Hexed aboraUy and toward the side of the arm. 



The proximal and distal joint faces on the first pinnular are very nearly parallel. 

 Also the distal joint face of the first pinnular shows here, usually, an almost straight 

 muscular articulation. Tins, however, is oriented at about a right angle to the 

 proximal facet of the ossicle, so that the transverse ridges of the two joint faces form 

 an anu'le of about 90°. The proximal facet of the second pinnular corresponds to 

 the distal facet of the first pinnular. In some comatulids this articulation is most 

 strongly developed on the adoral side. On contraction of the muscles in this articu- 

 lation the pinnule is Hexed ventrally. 



The distal end of the second pinnular also has a transverse ridge, and this again 

 makes an angle of do with the proximal face of the ossicle; here, therefore, it is a 

 dorsoventral crest. But the articulation between the second and third pinnulars is 

 differentiated from the two preceding by the appearance of small ventral muscle 

 bundles on either side of the ridge, and usually by the absence of the ligament pit 

 which, if it is present, marks the boundary for the dorsal development of the dorso- 

 il ere t . In these two features this pinnular articulation differs from a synarthry 

 that otherwise in its general character it rather strongly resembles. As is shown by 

 examples from the distal segments of the distal pinnules it may be considered as 

 derived from an ordinary muscular articulation through the disappearance of the 

 dorsal ligament. 



When asymmetry occurs in this and the following articulations it is the aboral 

 side of the joint face that becomes the more strongly developed. This is contrary to 

 the conditions found in the articulation between the first and second pinnulars (with 



some exceptions, as fur instance Metacrinus) . This stronger aboral development of 

 the articulations of the distal pinnule- expresses itself very strikingly in the devel- 

 opment of the combs in the ( 'omasteridae, and of the calcareous flanges protecting 

 the gonads in Austrometra, hometra, and certain Charitometridae. 



The ventral pits on either side of the dorsoventral crest for the reception of the 



le bundle- are Usually least distinct in the articulation between the second and 

 third pinnulars, where they may even be entirely lacking; but in the articulations 



