A MONOGRAPH OF THE EXISTING CRINOIDS 27 



extinct in the upper Carboniferous without leaving any descendants. The Articulata, 

 which according to Gislen may be traced back to Poteriocrinites, which in its turn is 

 descended from some group of Dendrocrinites, have independently followed a similar 

 course and developed the type usual in recent crinoids. Among the Jurassic comatu- 

 lids the muscle attachments are still relatively insignificant. In the radial muscular 

 articulations they may sometimes even be lacking, as in Pontiometra and Stephano- 

 metra; but this seems to be a condition of secondary origin. 



Gislen said that the ligamentary articulations would thus appear to be the oldest 

 arm flexors, which, through their action, have developed the transverse ridge. There- 

 fore the interarticular ligaments are the original antagonists of the dorsal ligaments. 

 The ligamentary articulations can be derived from the close sutural connections be- 

 tween the ossicles of the Cambrian, Ordovician, and Silurian crinoids. The role of 

 true muscles as arm flexors would be of later date, arising from the necessity for in- 

 creased and more rapid motion. The muscular articulations first reach their full 

 development in certain recent eleutherozoic comatulids, while other comatulids less 

 suited for swimming, many recent stalked crinoids, and the Permian to Jurassic forms, 

 have muscular attachments that are very modestly developed. 



LIGAMENTOUS ARTICULATIONS IN THE ARMS 



Gislen divided the articulations lacking striated muscle fibers into immovable 

 and movable ligamentary articulations. The immovable ligamentary articulations 

 he subdivided into (1) close synostosis (with a flat smooth joint face, as the junctures 

 between the radials) ; (2) syzygy; (3) pseudosyzygy; and (4) anchylosis. The movable 

 ligamentary articulations he divided into (1) those movable in all directions, and (2) 

 those movable in one plane only. Those movable in all directions he subdivided into 

 (a) articulations between the columnals of pentacrinites and comparable types, with 

 petaloid sculpture; (b) loose synostosis — the apposed joint faces somewhat concave, 

 without scultpure; and (c) pseudosyzygies as developed between the calyx plates in the 

 Flexibilia. Those movable in one plane only he divided into (a) synarthry (the 

 cryptosynarthry being a transition between the synarthry and synostosis) ; and (b) 

 ligamentary articulation, with at least three ligamentary pits (occurring in place of 

 syzygies in the Bathycrinidae, and according to Gislen between the radials and the 

 IBr! in Pontiometra and Stephanometra and perhaps also in burdigalocrinid stems; 

 this is the trifascial articulation of P. H. Carpenter). 



The articulations including striated muscle fibers Gislen divided into two cate- 

 gories: (a) those movable dorsoventrally, with one dorsal and two interarticular liga- 

 ments and two fairly strong muscle bundles — muscular articulations, sensu stricto; 

 these occur between the brachials, between the brachials and first pinnule segments, 

 and between the first two pinnule segments; and (b) those movable in a lateral direction 

 only, with no ligament fossae and usually small muscle fossae — pinnular articulations; 

 these occur between the pinnule segments from the second onward in the proximal 

 part of the arms. 



Gislen said that of the ligamentary articulations the oldest both ontogenetically 

 and phylogenetically is the synostosis. The synarthry can thus scarcely be derived 

 from the syzygy, or the syzygy from the synarthry ; both are differentiated varieties of 



