72 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



f. The distal borders of the interarticular ligament foss:e slope downward and In- 

 ward from the outer to the inner distal angles, or from the former to the rim 

 about the central canal. 

 g l . The interarticular ligament fossa/ are triangular, their inner sides forming a 



straight line from the outer distal angle to the rim about the central canal. 

 ft 1 . The muscular foss;i> are very narrowly linear, forming narrow bauds of uni- 

 form width along the distal borders of the interarticular ligament fossae. 



Ptilometra mulleri. 



h'. The muscular fossa; are larger, triangular or trapezoidal. 

 i 1 . The outer borders of the interarticular ligament t'ossse are very strongly con- 

 vergent, so that the distance across (he distal border of the articular face 

 is much less than half the length of the transverse ridge; the muscular 

 foss;e are relatively high, their maximum (inner) height being twice their 



width Cololioinetrii ilixi'iilor. 



i'. The outer sides of the iuterarticular ligament fossae are much less converg- 

 ent, so that the distance across the distal border of the articular face is 

 equal to three-quarters of the length of the transverse ridge; the muscular 

 fossie are very low, their maximum (inner) height being much less than 



their width OUgometra serripinna. 



g'. The iuterarticular ligament fossa? are four sided, or have the distal border very 



broadly rounded ; they are never triangular. 



/i 1 . The muscular fnss.r are broadly rounded distnlly and outwardly, tapering to a 

 point inwardly and proximally ; their border is everywhere evenly curved, 

 its continuity not being interrupted by the interarticular ligament fossa;. 



f. The intermuscular notch has parallel sides Amphimetra papuensis. 



f. The intermuscular notch is at least twice as broad proximally as distally. 



Heterumetm phililterti. 



It 2 . The muscular fosste are narrow and more or less band-like; the distal edge 

 of the interarticular ligament foss;? always cuts away more or less of their 

 proximal border. 



i 1 . The intermuscular furrow is suddenly broadened out proximally just over 

 the central canal, being constricted between the outer portions of the 

 dorsal ligament fossa-. 

 j l . The interarticular ligament fossa? are unusually narrow, being more than 



twice as high as the minimum width Hcteroinetra rei/nau/lii. 



f. The interarticular ligament tossre are much broader, being not more than 



half again as high as the minimum width Craspedometra acuticirra. 



f. The sides of the intermuscular furrow are parallel, or diverge regularly 



from the base. 

 f. The sides of the intermuscular furrow diverge rapidly from the base 



outward Lamprometra protectus. 



f. The sides of the intermuscular furrow are parallel. 



k l . The intermuscular furrow is broad, with a median carinate ridge. 



Maria metra subcarinata. 

 A-'. The intermuscular furrow is narrow, with no median carination. 



f. The distal borders of the interarticular ligament fossw are more or 

 less regularly convex ; the muscular fossa; extend inward and down- 

 ward so that their proximal apex is half way between the maximum 

 height of the iuterarticular ligament fossae and the base of the 



intermuscular furrow Amphimetra ensifer. 



? = . The distal borders of the interarticular ligament fossiv are flattened; 

 the inner ends of the muscular fossa; do not pass beyond the inner 

 distal angles of the interarticular ligament fossae. 



