MONOGRAPH OF THE EXISTING CRINOIDS. O 



The articular faces of adjacent radials are separated from each other later- 

 ally by a narrow groove, which usually is closed distally by the apposition of 

 the outer edges of the muscular fossae. 



The plane of the joint face as a whole typically makes an angle of about 45° 

 with the dorsoventral axis of the animal; if the radials become nearly or quite 

 horizontal this angle decreases so that the plane of the joint face becomes almost 

 or quite parallel to the dorsoventral axis; but conversely it inclines more to the 

 horizontal if the radials slant upward and outward as in the pentacrinites. The 

 jjroximal portions of the muscular fossae slope inward so as to make an angle 

 of about 90° with each other. 



There are two lines of divergence from this generalized type — (1) there may 

 be an increase in flexibility at the articulation, inducing a corresponding modi- 

 fication of the joint face, or (2) there may be a gradual decrease in flexibility, 

 reaching a maximum in a practically immovable union. 



The first line of divei-gence is rare and is only found among certain 5 or 10 armed 

 species belonging to various macrophreate groups, such as the Pentametrocrinidse 

 (figs. 114, p. 67, and 1006, pi. 4), the Atelecrinidse, and the Heliometrinae (figs. 

 95, 97, p. 62, and 997, 999, 1000, pi. 4) and in the young of certain other types. 

 It is characterized by a great increase in the size of the muscular fossfe, coupled 

 with more or less reduction in the size of tlie interarticular ligament fossae, and 

 a marked deepening in the sculpture of the joint face as a whole. 



Instances of the second line of divergence are much more common, some 

 progress along this line having been made by almost all of the oligophreate and 

 by many of the macrophreate t}'pes. The first modification noticeable is a dimi- 

 nution in the size of the muscular fossa?, which may become narrow laterally as 

 in the Thalassometrida; (figs. 69-75, p. 43) and in the Charitometridse (fig. 76, 

 p. 53, and figs. 984-993, pi. 3), or very short as in most of the other groups 

 (figs. 25-38, p. 20). Shortening of the muscular fossae is correlated with a de- 

 crease in the size of the dorsal ligament fossa, which becomes transversely semi- 

 elliptical ; an increase in the size of the interarticular ligament f ossw, which almost 

 always develop an obtuse angle on their inner margins (figs. 39-50, p. 26) : a 

 progressive shallowness of the joint face sculpture, the intermuscular ridge at 

 the same time becoming an intermuscular furrow; and a flattening and eversion 

 of the joint face as a whole, so that it becomes more and more nearly parallel 

 with the dorsoventral axis of the animal. This line of development leads to a 

 joint face more or less parallel with the dorsoventral axis of the animal, showing 

 a narrow semielliptical dorsal ligament fossa with a transversely linear shallow 

 ligament pit, two approximately square interarticular ligament fossae internally 

 separated by a shallow furrow about as wide as the central canal, and very nar- 

 rowly linear muscular fossae, or even no muscular fossae at all. The relief of 

 the joint face is now so shallow that the rim or platform about the central canal 

 and along the distal (ventral) border of the transverse ridge is not evident. 



In connection with the following detailed account of the radial articular faces 

 of the comatulids I have included reproductions of all the figures which have been 

 published by previous authors. It has seemed best not to attempt to include de- 



