182 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



and part 1, fig. 32, p. 71) the course of the transverse ridge is oblique, normally 

 making an angle of about 45 with the dorsoventral plane of the joint face, the 



interarticular ligament and 

 the muscular fossae on one 

 side of the median line con- 

 sequently being crowded 

 and those on the opposite 

 side correspondingly ex- 

 tended, and viewed dorsally 

 the joint face is seen to 

 cross the arm at more or less 

 of an angle (typically about 

 45) to the longitudinal 

 axis. 



In an external dorsal 

 view of a postradial series of 

 ossicles a straight muscular 

 articulation may usually be 

 at once distinguished by 

 its straight course across 

 the arm and by having the 

 two points of contact mark- 

 ing the ends of the apposed 

 fulcral ridges equidistant 

 from the median dorsal 

 line. An oblique mus- 

 cular articulation, on the 

 other hand, runs diagonally 

 across the arms, most com- 

 monly at an angle of from 

 30 to 45, and the two 

 points of contact are one 

 dorsolateral, the other ven- 

 trolateral (part 1, fig. 30, 

 p. 71). 



Two oblique muscular 

 articulations following each 

 other are always reversed, 

 and this alternation is the 

 invariable rule. If the ful- 

 cral ridge of one oblique 

 muscular articulation crosses 

 the joint face from a left 

 ventrolateral to a right 

 dorsolateral position, the 



227 



PIG. 227. LATERAL VIEW OF SPECIMEN OF PERISSOMETRA SELENE. 



