No. 514] ARTICULATIONS OF CRINOIDS 581 



cular articulation were not there, but the two joints con- 

 nected by it merely a single joint. 



Of the non-muscular articulations, the synarthry occurs 

 in the proximal part of the arm, the last synarthry im- 

 mediately preceding the first oblique muscular, and im- 

 mediately succeeding the last straight muscular articula 

 tion; all the non-muscular articulations succeeding the 

 first oblique muscular articulation are always syzygies. 

 In the simple arms of the Pentametrocrinida- we find a 

 straight muscular articulation, a synarthry, and then a 

 series of oblique muscular articulations, interspersed 

 with occasional syzygies. In this family the first 

 brachial immediately follows the radial; but in all the 

 other comatulids and in the recent species of the IVn 

 tacrinitidae (except in the genus Metarrinus) the lir<t 

 brachial is separated from the radial by one or more 

 interpolated division series, each composed of a redupli- 

 cation of the first two brachials interpolated between the 

 primitive first brachial and the radial. In these, how 

 ever, the structure is the same; a series of synarthries 

 alternating with straight muscular articulations occurs up 

 to the first oblique muscular articulation, beyond which 

 are found only oblique muscular articulations and 

 syzygies. This is the primitive arrangement of the 

 comatulid and pentacrinite arm, no matter how many 

 times division may occur; but in certain specialized 

 types, as Endoxocrinus and the Zygometrida?, one or more 

 of the synarthries may be secondarily replaced by 



muscular articulations are morphologically radically dif- 

 ferent from muscular articulations; and (2) that there 

 is a distinct interrelation between the two types of mus- 

 cular articulations and the two types of non-muscular 

 articulations; that is, that proximal to the first oblique 

 muscular articulation only straight muscular articula- 

 tions and synarthries are found, while distal to the first 

 oblique muscular articulation occur only ohliquc muscular 



