MONOGRAPH OF THE EXISTING CKINOIDS. 27 



The lateral borders of the interarticular ligament fossae are rather strongly 

 concave, the curve being somewhat stronger in the distal than in the proximal 

 halves. The outer distal corners of the muscular fossa? are very nearly directly 

 above the ends of the transverse ridge. \^iewed perpendicularly the interarticular 

 ligament fossae appear about as high as the distance from the end of the transverse 

 ridge to the central canal. The lateral edges are concave and, as a whole, slope 

 slightly inward ; the distal borders are more or less convex with the chord of the 

 convexity slightly inclined, so that the inner distal corners are about one-fifth of 

 the length of the lateral border below the outer. The inner distal angles are 

 broadly rounded and the inner borders are S-shaped, the distal lobe of the S being 

 formed by the inner corners and the lower by a broadly rounded notch of about 

 the same size, which inwardly reaches the distal margin of the central canal. 

 There is no raised rim about the central canal. The furrow between the inter- 

 articular ligament fossae is therefore of a broad double S or gourd shape, the 

 diameter of the expanded basal portion being approximately equal to the width 

 of the interarticular ligament fossae at that point. 



The muscular fossas are narrow, with parallel sides; outwardly they project 

 in the form of an acute angle beyond the outer distal corners of the interarticular 

 ligament fossas; inwardly they run to just beyond the inner distal angles of the 

 interarticular ligament fossas, where they taper to a point. The distance between 

 the inner sides of the muscular fosses as they curve downward is about equal to 

 their own width and is considerably less than the distance between the inner distal 

 corners of the interarticular ligament fossas just below. 



The space between the radials ventrally is filled to the height of the distal 

 edges of the muscular fossae with a spongy calcareous mass, almost plane on the 

 ventral surface, with a small central canal. 



Diameter of radial pentagon at base, 5 mm. ; height of articular face (measured 

 along the inclination), 2.5 mm.; width, at transverse ridge, 2.8 mm.; distance from 

 center of rosette to middle of dorsal outer edge of radial, 2.3 mm. ; distance from 

 center of ventral face of radial pentagon to middle of ventral outer edge of 

 radial, 1.4 mm. ; to apex of interradial angle, 1.8 mm. 



HETEROMETRA 



Figs. 45, 46, p. 26. 



The edges of the dorsal ligament fossa lie in a plane making only a small 

 angle with the dorsoventral axis. From the ends of the transverse ridge onward 

 the sides of the interarticular ligament and muscular fossae together form a strongly 

 concave line, so that the outer ends of the muscular fossae are not much within 

 the ends of the transverse ridge, being in about the same plane in reference to the 

 dorsoventral axis as the inner border of the central canal. 



The dorsal ligament fossa is somewhat over twice as broad as long; the 

 proximal border is slightly flattened. 



