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 

 fi-om 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 



Fio. 227. — Lateral view of specimen of Perissometea selene. 



