56; BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Height of articular face (measured along the inclination), 1.9 mm.; width, 

 at transverse ridge, 1.5 mm.; distance from center of rosette to middle of outer 

 dorsal edge of radial, 0.9 mm. 



PEROMETRINiE. 



The radials project much more than usual from the centrodorsal, so that the 

 proximal border of the dorsal ligament fossae is considerably beyond the periphery 

 of the ventral rim of the centrodorsal. 



The exposed dorsal surface of the radials is slightly concave in profile instead 

 of, as usual, more or less convex. 



The radial articular faces are broader than high and are inclined inward at 

 an angle of approximately 60° with the dorsoventral axis. 



They are entirely, though slightly, separated from each other interradially. 



All the joint face elements lie in the same plane. The muscular fossae are not 

 greatly larger than the interarticular ligament fossie, which are about the same 

 height as the dorsal ligament fossa. The proximal border of the dorsal ligament 

 fossa is slightly flattened. The general direction of the distal border of the inter- 

 articular ligament fossae is upward and outward, at an angle of 45° to the trans- 

 verse ridge. The distal border of the interarticular ligament fossae is, owing to 

 the comparatively small amount of excavation of the inner portion of the joint 

 face, rather low; it may bear a supplementary boss at its inner end; the outer 

 border may be straight, slightly convex, or moderately concave. The muscular 

 fossae are separated interiorly by a broad low intermuscular septum with parallel 

 sides, which may be more or less concave along its summit. The intermuscular 

 notch is broad, not very deep, and well rounded i^roximallj'. 



The inner sides of the radials slope inward, so that the inner part of the radial 

 pentagon is funnel-shaped instead of a hollow cylinder as usual, and the free 

 central canal is therefore comparatively small. Tliere is more or less of a deposi- 

 tion of calcareous matter within the radial pentagon and on the ventral surface 

 of the rosette. 



PEROMETRA DIOMEDEjE. 



Figs. 82-S4, p. 53. 



The dorsal ligament fossa is narrow, about three times as broad as high, 

 with the proximal border semielliptical or slightly flattened. The ligament pit 

 is reniform or broadly oval, somewhat larger than the central canal but not 

 strongly marked. 



Each articular face is entirely separated from its neighbors by a shallow 

 furrow, which at the ends of the transverse ridges is about as broad as the central 

 canal, becoming slightly broader between the interarticular ligament fossa; and 

 considerably narrower between the muscular fossae. 



The interarticular ligament fossa? are somewhat higher than the dorsal liga- 

 ment fossa. Their outer border is more or less concave and makes a general 



