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THE AMERICAN NATURALIST [Vol. XLIII 



quent depression dorsally of that half of the transverse 

 ridge, which is compensated by a ventral depression of 

 the opposite half ; this alteration in one half of the trans- 

 verse ridge is a necessary consequent of any alteration 

 in direction of the other half, for the transverse ridge is 

 the fulcrum upon which the bending of the arm occurs, 

 and the fulcrum must always be a straight line to admit 

 of any motion at all. Although primarily pinnulate, in 

 certain rare cases oblique muscular articulations are 

 sometimes found non-pinnulate, as in Atelecrinus, 

 Hypalometra, Gyllometra, and Perometra; but this is a 

 purely secondary condition, and one peculiarly prone to 

 reversion, showing it to be somewhat unstable. In all 

 cases, the position of the first oblique muscular articula- 

 tion is the second articulation beyond the last straight 

 muscular articulation of the arm. The oblique muscular 

 articulations always alternate in the position of their 

 diagonal transverse ridges, and the transverse ridges of 

 succeeding joints form angles of approximately 90° with 

 each other; therefore, a single brachial has proximally 

 an articular face with the transverse ridge from a left 

 ventro-lateral to a right dorso-lateral point, and distally 

 an articular face with the transverse ridge running from 

 a left dorso-lateral to a right ventro-lateral position. 

 The pinnule socket always occurs on the side on which 

 the end of the transverse ridge is dorso-lateral in posi- 

 tion ; hence, pinnules occur on alternate sides of the arm 

 at succeeding articulations. In reality, of course, the 

 alternation of the pinnules is the fundamental cause of 

 the alternation in the direction of the transverse ridge, 

 but, from the absence of pinnules on oblique muscular 

 articulations in certain recent types, it is more convenient 

 to speak of it as if the reverse were the case. 



Non-muscular articulations are never doubled, are 

 never pinnulate, and moreover, never affect the pinnula- 

 tion in any way ; the pinnule on the next succeeding mus- 

 cular articulation is on the opposite side from that of the 

 preceding muscular articulation, just as if the non-mus- 



