24 BULLETIN 82, UNITED STATES NATIONAL. MUSEUM. 



high narrow rim about the lateral and distal edges of the central canal, which is 

 continued laterally in the shape of a high border along the transverse ridge more 

 or less broad interiorly but tapering gradually to a point at either end of the 

 transverse ridge. The presence of a riin about the central canal, therefore, is 

 due to the amount of action between the radials and the IBr, and the correlated 

 amount of excavation, and has no systematic significance of itself apart from this. 

 The depth of the dorsal ligament fossa and the depth of the ligament pit is in 

 general proportionate to the depth of the interarticular ligament fossae and the 

 correlated size of the muscular fossae. 



The elements of the joint face on either side of the transverse ridge are usually 

 in the same plane ; if there is any difference, the plane of the dorsal ligament fossa 

 makes a slightly greater angle with the dorsoventral axis than the plane of the 

 distal elements. 



The dorsal ligament fossa is large, with the proximal border well rounded 

 and never flattened. The ligament pit is strongly marked, reniform, oval, or cir- 

 cular, usually about the size of the central canal, which is broadly oval and 

 moderate in size. 



The joint face elements distal to the transverse ridge and the central canal 

 are separated interiorly by a broad and shallow intermuscular furrow as broad 

 as to very much broader than the central canal, which may have parallel sides, 

 may be very broad proximally and strongly constricted between the muscular 

 fossae, or may increase gradually in width distally. 



The interarticular ligament fossae are usually considerably higher than the 

 dorsal ligament fossa, higher than broad, irregularly quadrate or oblong. They 

 are usually deeply hollowed, at least exteriorly. Their outer edges are more or 

 less strongly concave, at least basally; their distal edges are straight, or more or 

 less strongly convex, and may be parallel to the transverse ridge, but usually run 

 more or less inward and downward, sometimes almost directly from the outer 

 distal angles to the central canal. The inner distal angles, however, are never 

 obliterated, though they may be very broadly rounded. 



The muscular fossae are variable in size, but are always smaller than the 

 interarticular ligament fossae. They may be greatly reduced and narrowly linear, 

 or even apparently absent, but are usually comparatively broad exteriorly, gradu- 

 ally narrowing to a point just at, or somewhat within, the distal angles of the 

 interarticular ligament fossae ; they may have two slight ridges across them parallel 

 to their distal margin which probably represent the limits of annual growth before 

 maturity or full size is reached. Their outer distal angle always projects strongly 

 outward beyond the outer edge of the interarticular ligament fossae. 



The ventral face of the radial pentagon is pentagonal or bluntly stellate. In 

 small 10-armed species or in 10-armed young it is sometimes rather deeply stellate, 

 so that the interradial radius may be twice as long as the radial radius. 



The ventral plug is well developed and has a flat or slightly concave surface, 

 which is nearly or quite on a level with the distal edges of the muscular fossae. It 

 almost or quite conceals the inner surface of the radials. The radial canals are 

 short, narrow, or quite obliterated ; there is rarely a free central canal. 



