32 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



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

 gently convex proximally but turning up more sharply at either end to the ends 

 of the transverse ridge. The dorsal ligament fossae do not communicate with each 

 other interradially. but the transverse ridges are in contact. Interradially the 

 interarticular ligament and muscular fossae of adjacent joint faces are separated 

 by a very narrow V- or U-shaped gap which reaches down almost to the transverse 

 ridge. 



The height of the interarticular ligament fossae measured from the proximal 

 border of the transverse ridge is approximately equal to the height of the dorsal 

 ligament fossa measured from the same point. The outer edges of the inter- 

 articular ligament fossae are nearly straight; the distal edges are more or less, 

 usually strongly, convex, and the inner edges are nearly or quite straight and run, 

 converging slightly, to a point just above the lateral borders of the central canal, 

 then turn inward and meet each other in a straight line just beyond the central 

 canal. 



The broadly U-shaped space between the interarticular ligament fossae, the 

 sides of which flare slightly, is almost or quite equal in area to that portion of the 

 interarticular ligament fossae beyond the central canal, and is usually not greatly 

 different from it in shape. 



The distal borders of the interarticular ligament fossae may be quite semi- 

 circular, so that no inner or outer corners are evident, but they are usually not 

 quite so convex, in which case the chord of the curve is usually horizontal that 

 is, parallel to the transverse ridge- 



There is no rim about the central canal. 



The muscular fossae are small and narrow ; their greatest width is about equal 

 to the transverse diameter of the central canal, or is rather less. The outer margin, 

 which runs in the same direction as that of the interarticular ligament fossae, is 

 more or less convex, sometimes almost semicircular. The distal edges run parallel 

 to those of the interarticular ligament fossae, gradually turning downward and 

 approaching the edge of those fossae, making with them a sharp angle with the 

 apex about halfway distant between the distal border of the interarticular liga- 

 ment fossae and that of the central canal. 



The free borders of the muscular fossae are approximately semicircular, and 

 the fossae themselves are crescentic, but the crescent is truncated at its broadest 

 part, just over the outer distal angles of the interarticular ligament fossae. 



The shallow furrow beween the inner edges of the interarticular ligament and 

 muscular fossae is in general slightly less broad than the dorsal canal. Its sides 

 are approximately parallel but are slightly further apart below the inner ends 

 of the muscular fossae; through the center of this furrow there runs a sharp high 

 longitudinal ridge, dividing it into two parallel furrows. 



The ventral surface of the radial pentagon is covered with a porous cal- 

 careous mass which is nearly flat on the surface and reaches to the height of 

 the highest portions of the muscular fossae; it contains a rather small central 

 perforation. 



