68 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Diameter of radial pentagon at base, 4.7 mm. ; height of articular face (meas- 

 ured along the inclination), 2.3 mm.; width, at transverse ridge, 2.8 mm.; distance 

 from center of rosette to middle of dorsal outer edge of radial, 2 mm. ; diameter 

 of ventral interarticular space, 1.7 mm. 



PENTAMETROCEINIDiE. 



The radial articular faces are widely separated from each other. 



The dorsal ligament fossae are broadly rounded proximally. 



The interarticular ligament fossw are lower than the dorsal ligament fossa, 

 strongly convex exteriorly, the very low and narrow septum forming the distal 

 margin making, at least interiorly, an angle of 45° with the transverse ridge. It 

 may curve downward so that it makes but a small angle with the transverse ridge 

 exteriorly. 



The muscular fossae are about twice the height of the interarticular ligament 

 fossae, and are very broadly rounded. The intermuscular septum is high and 

 narrow. 



The excavation of the joint face is very deep, and the calcareous lamina form- 

 ing the various joint face elements are very thin. 



PENTAMETROCEINUS JAPONICUS. 

 Figs. 114, 115, p. 67. 



The dorsal ligament fossa is very narrow, about five times as broad as long, its 

 proximal border from the ends of the transverse ridge forming a very regular 

 half ellipse. The fossa is very deep, and there is no definite ligament pit. 



The radial articular faces are entirely and rather widely separated from each 

 other interradially by deep grooves. The ends of the transverse ridges of adjacent 

 joint faces are separated bj' a space about as broad as the dorsoventral (shorter) 

 diameter of the central canal. 



The lateral borders of the interarticular ligament fossse make an angle of 

 about 45° with the transverse ridge, basally passing over into the curve of the 

 lateral ends of the dorsal ligament fossa around the ends of the transverse ridge. 

 In length they are about two-thirds of the distance from the central canal to the 

 ends of the transverse ridge. From the distal angle of the interarticular ligament 

 fossae the distal edge, which is a narrow and low, but prominent, ridge, runs 

 diagonally downward in a straight line, which makes a somewhat less angle with 

 the transverse ridge than does the outer border, to the prominent raised rim about 

 the central canal which it joins at a level just above that of the distal edge of the 

 latter. 



The muscular fossae lean inward, malring a greater angle with the dorsoventral 

 axis than do the proximal elements of the joint face. When viewed along the line 

 of the dorsoventral axis they are seen to have the lower and inner borders (the 

 latter on the intermuscular septum) at right angles to each other and of the same 

 length. The outer border is a broad curve approaching a quadrant of a circle. 



