MONOGRAPH OF THE EXISTING CRINOIDS. 45 



The distal borders of the interarticular ligament fossas arise on the dorsal side 

 of the rim about the central canal and run outwardly at an angle of about 45 with 

 the transverse ridge to the outer edge of the joint face. The distal angle may be 

 somewhat rounded. 



The muscular fossae are roughly rhombic in shape, the greater diameter being 

 about twice the lesser. The distal and outer sides are convex and the inner and 

 outer distal angles are well rounded, especially the former. The inner sides, inward 

 from the inner distal angles, are parallel and are well separated by a shallow furrow 

 equal in width to about half the horizontal diameter of the central canal, which 

 bears in the median line a narrow longitudinal ridge. In the proximal portion the 

 floor of this shallow furrow gradually rises and at the same time the sides gradu- 

 ally diverge, so that it merges, by a sort of triangular base, into the rim about the 

 central canal. 



The ventral surface of the radial pentagon is strongly concave; the depressed 

 center is occupied by a plug of porous calcareous deposit; the ventral surface of 

 the radials is longitudinally striate. 



Diameter of radial pentagon at base, 4 mm. ; height of articular face (measured 

 along the inclination), 2.8 mm.; width, at transverse ridge, 3 mm.; distance from 

 center of the rosette to middle of dorsal outer edge of radial, 2 mm. ; diameter of 

 ventral interarticular space, 2 mm. 



THALASSOMETRIN.*:. 

 THAI,ASSOMETRA VILLOSA. 



Fig. 71, p. 43. 



The portion of the radial articular face distal to the transverse ridge lies in a 

 plane making an angle of approximately 60 with the dorsoventral axis of the 

 animal until the distal half of the muscular fossae is reached ; these are parallel 

 with the dorsoventral axis, and at the tip turn slightly outward. The plane touch- 

 ing the edges of the dorsal ligament fossa lies at an obtuse angle with the plane 

 of the interarticular ligament fossae, and makes only a small angle with the dorso- 

 ventral axis. The adjacent radial articular faces are in close contact with each 

 other except distally, where the eversion of the tips of the muscular fossae cause 

 the formation of narrow V-shaped gaps between them. 



The dorsal ligament fossa? are about two and one-half times as broad as long, 

 moderate^ and evenly curved proximally, but turning up rather abruptly at the 

 ends. They are entirely separated from their neighbors, but the ends of the trans- 

 verse ridges are just in contact. 



The rim about the central canal is strong and very prominent, as high as the 

 distal part of the transverse ridge. The distal border of the interarticular liga- 

 ment fossse is formed by a sharp ridge considerably lower than the rim about the 

 central canal which arises on the side of that rim midway between its distal center 

 and the place where it joins the transverse ridge and runs, making an angle of 60 

 with the transverse ridge, to the edge of the joint face. 



