MONOGRAPH OF THE EXISTING CRINOIDS. 51 



ment fossae may make an angle of 45 with the transverse ridge, are usually 

 more nearly horizontal. They may carry a supplementary boss at the inner angles. 

 The outer borders are usually slightly convex; the muscular fossae are broadly 

 rounded, quadrilateral, usually broader than high, though they may be slightly 

 higher than broad if the distal border of the interarticular ligament fossae is 

 very oblique. Their lateral edges project slightly, if at all, beyond those of the 

 interarticular ligament fossae and maintain the same general direction. The 

 muscular fossae are separated interiorly by a broad intermuscular septum which 

 is more or less concave at its summit and is broader proximally than distally. 

 The intermuscular notch is shallow and broadly rounded proximally. 



ANTEDON PETASUS. 

 Fig. 78, p. 53. 



The dorsal ligament fossa is about two and one-half times as broad as high, 

 with the proximal border semielliptical. The ligament pit is shallow, somewhat 

 larger than the central canal, with which it is confluent distally. 



The outer edge of the interarticular ligament fossae is nearly or quite straight 

 and makes an angle of about 60 or 70 with the transverse ridge. It is about 

 half as long as the distance from the end of the transverse ridge to the edge of 

 the central canal. From the distal corner the distal edge of the interarticular 

 ligament fossae slopes inward, making a small angle with the transverse ridge 

 and joining the intermuscular septum at a level about as far above the level of 

 the distal border of the central canal as the median diameter of the canal. This 

 distal border of the interarticular ligament fossae is straight or slightly convex ; 

 in profile it is seen to be a high ridge rapidly decreasing in height outwardly, 

 with a prominent broad, shallow, rounded notch about halfway between its inner 

 and outer ends. Its inner portion bears on the outer side a prominent supple- 

 mentary muscle plate which extends outward about halfway to the outer end. 



The intermuscular septum is very broad, just over the central canal being 

 nearly as broad as the diameter of that canal. It narrows distally and beyond 

 the supplementary muscle plates is only about half as wide, with parallel sides. 

 It carries a broad, shallow median groove, which, just over the central canal, is 

 more or less expanded and somewhat deepened. 



The muscular fossae are about as high as the dorsal ligament fossae; the 

 outer side is broadly rounded ; the intermuscular notch is well rounded, its sides 

 making with each other an angle of approximately 90. 



The elements of the joint face distal to the transverse ridge make, when 

 viewed in profile, an angle of about 90 with each other. 



There is little or no calcareous deposit within the ventral portion of the 

 radial pentagon. 



Height of articular face (measured along the inclination), 1.4 mm.; width, 

 at transverse ridge, 2 mm. 



