50 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



that the muscular fossse are only separated from the very large central cavity 

 by thin calcareous lamina, which are quite different from the more or less thick 

 wedges seen in the Oligophreata ; (3) the joint face elements distal to the transverse 

 verse ridge are strongly excavated, especially interiorly, so that they meet in the 

 median dorsoventral line at an angle of usually 90°. This results in the forma- 

 tion of a sort of tube or tunnel about the central canal, for the rim about the 

 central canal and the associated structures are always on the same articular level 

 as the transverse ridge, no matter how deep the excavation of the joint face may be. 



The dorsal ligament fossa usually equals the interarticular ligament fossae in 

 height, and its proximal border is rather strongly convex, sometimes even rounded- 

 triangular. It is deeply excavated, so that the ligament pit is usually rendered 

 more or less obsolete, sometimes being evident only as a shallow and broad groove 

 on the lower (dorsal) side of the transverse ridge. 



The interarticular ligament fossas are usually triangular with their inner 

 angles truncated, but their distal border rarely makes so great an angle as 60° 

 with the transverse ridge Frequently the distal border inwardly makes an angle 

 of from 45° to 60° with the transverse ridge, but gradually curves outward so that 

 outwardly it is almost or quite parallel to the transverse ridge, or even slopes more 

 or less downward. 



The septum bounding the interarticular ligament fossse anteriorly is inwardly 

 very high and rather narrow. In profile it is seen to decrease verj^ rapidly in 

 height at first, then to make a broad curve and to decrease gradually and evenly 

 to its outer end; in a few forms this septum is so reduced as only to be indicated 

 by a low narrow ridge, or it may entirely disappear before the outer edge of the 

 joint face is reached. The outer edge of the interarticular ligament fossae is 

 usually straight, but it may be strongly concave or strongly convex. 



The muscular fossie are always larger, usually much larger, than the inter- 

 articular ligament fossae, and they are always very thin and broadly rounded; 

 they are separated interiorly by a septum which is usually narrow and high, and 

 their tips are separated by a more or less developed intermuscular notch. 



The central canal is large and Avell rounded, and is bordered b}' a high 

 narrow rim. 



ANTEDONID^. 



ANTED0NIN.aE. 



The articular faces of the radials are broader than high. They are inclined 

 inward at an angle of approximately 60° with the dorsoventral axis and are entirely, 

 though slightly, separated from each other interradially. All of the joint face 

 elements lie approximately in the same plane. The muscular fossae are but 

 slightly larger than the interarticular ligament fossae, which are of about the 

 same height as the dorsal ligament fossae. The proximal border of the dorsal 

 ligament fossae is strongly convex. The distal borders of the interarticular liga- 



