72 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



f. The distal borders of the iiitei-articular 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^. The interartieular ligament fossa" are triangular, their inner sides forming a 

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

 h^. The muscular fossfe are very narrowly linear, forming narrow bands of uni- 

 form width along the distal borders of the interartieular ligament fossse. 



Ptilometra miilleri. 

 h'. The muscular fossse are larger, triangular or trapezoidal. 



t". The outer borders of the interartieular ligament fossae are very strongly con- 

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

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

 fossre are relatively high, their ma.ximum (inner) height being twice their 



width Colohometra discolor. 



i'. The outer sides of the intei-articular ligament fossse 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 

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



their width OUfionietra serripmna. 



g'. The interartieular ligament fosste are four sided, or have the distal border very 



broadly rounde<l; they are never triangular. 



/('. The muscular foss.'e :ire liroaiUy njundi-rt distally and dUtwardly, tapering to a 



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



its continuity not being interrupted by the interartieular ligament fossne. 



C. The intermuscular notch has parallel sides Amphimetra papuensis. 



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



Seteromeira philiierti. 



h'. The muscular fossse are narrow and more or less band-like; the distal edge 



of the interartieular ligament fossse always cuts away more or leas of their 



proximal border. 



iS The intermuscular furrow is suddenly broadened out proximally just over 



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



dorsal ligament fossa*. 



f. The interartieular ligament fossfe are unusually narrow, being more than 



twice as high as the miniuunn width Hcterotnetra reynaudii. 



p. The interartieular ligament fossse are much broader, being not more than 



half again as high as the minimum width Craspedometra acuticirra. 



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

 from the base. 

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



outward Lamprometra protectus. 



i". The sides of the intermuscular furrow are parallel. 



fc". The intermuscular furrow is broad, with a median carinate ridge. 



Mariametra suhcarinata. 

 ]{'. The intermuscular furrow is narrow, with no median carination. 



l^. The distal borders of the interartieular ligament fossre are more or 

 less regularly convex ; the nniseular fossfe extend inward and downi- 

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

 height of the interartieular ligament fossse and the base of the 



Intermuscular furrow Amphimetra ensifer. 



/^ The distal liorders of the interartieular ligament fossa? are flattened; 

 the inner ends of the muscular fossse do not pass beyond the Inner 

 distal angles of the interartieular ligament fossse. 



