28 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The fossae distal to the transverse ridge are separated intcrradially by a narrow 

 lenticular gap running from near the ends of the transverse ridge to the outer 

 angles of the muscular fossae. 



The outer border of the interarticular ligament fossa 1 is rather strongly con- 

 cave, and has a general trend inward, so that the outer distal angles of the inter- 

 articular ligament fossae lie over a point about halfway between the central canal 

 and the ends of the transverse ridge. The distal border is slightly convex and runs 

 inward and downward at an angle of approximately 45 : inwardly the edges 

 make a rather broad curve and the inner edges run parallel to the outer (thus 

 diverging) until the level of the distnl border of the central canal is reached, 

 when they turn in a rather sharp but resrular curve and reach the central canal 

 just below its distal edge. The shallow groove between the interarticular ligament 

 fossae is. at the inner distal angles of the latter, about as wide as the diameter of 

 the interarticular ligament fossa? at that point. 



The muscular fossae are narrow, about as wide as the space between them 

 (which is less than that between the distal inner angles of the interarticular liga- 

 ment fossae). The proximal and distal borders are parallel; inwardly they turn 

 downward rather abruptly, their outer borders running parallel or diverging very 

 slightly, meeting the border of the interarticular ligament fossae at the inner angle 

 in a sharp point: outwardly the muscular fossae project beyond the outer distal 

 corners of the interarticular ligament fossae in the form of a rounded triangle the 

 length of which is about equal to the length of the distal edge of the interarticular 

 ligament fossae. 



The space between the radinls ventrally is filled to the level of the distal 

 edges of the muscular fossa 1 with a porous calcareous filling marked on the sur- 

 face with indistinct radiating lines or papilla? and with a comparatively large 

 central canal, in comparison with the relatively small size of the ventral face of 

 the radial pentagon. 



Diameter of radial pentagon at base, o mm.: height of articular face (meas- 

 ured along the inclination), 2.3 mm.; width, at transverse ridge, 2.5 mm.; distance 

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

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

 of radial, 0.8 mm. ; to apex of interradial angle, 1.2 mm. 



HETEROMETRA PHILIBERTI. 

 Figs. 34-36, IX 20. 



The dorsal ligament fossa is about twice as broad as high, nearly semicir- 

 cular. The ligament pit is circular, somewhat larger than the transversely oval 

 central canal. 



The transverse ridges of adjacent joint faces join in the interradial angles, 

 but the dorsal ligament fossae are widely free. The interarticular ligament fossae 

 are separated in the interradial angles by a narrow V-shaped opening, which is 

 bridged over distally by the apposition of the muscular fossae. 



