AXIAL SKELETON OF THE OSTRICH. 441 



The preaxial or ventral margin of the rib is generally rounded, the dorsal margin 

 sharp. 



There is no sudden inflection in the rib which can be called an " angle ;" nor does its 

 dorsal margin develop a rounded prominence as was the case in the second rib. 



The entire preaxial margin is at first (i. e. proximally) concave, and afterwards very 

 gently convex. 



The entire dorsal margin is at first very slightly convex, and then concave in a little 

 more marked degree. 



The Fourth Bib. 



Though the increase in length of the fourth rib (fig. 75, iv & iv') is much less 

 than we meet with in proceeding from the second to the third, yet it exceeds the last- 

 mentioned very considerably in bulk and massiveness, though it is less, if not much less, 

 than half as long again (fig. 1 , iv). 



The fourth does not end freely, but articulates distally by an expanded concave 

 surface, with the second sternal rib ; it is attached dorsally to the twenty-first vertebra. 



The increase in length of the capitulum of the rib (as compared with that of the 

 third rib) is considerably greater than that of the tuberculum. Concomitantly with 

 this, it seems rather to be the capitulum which continues on the general curve of the 

 rib, from which the tuberculum diverges obliquely dorsad. 



The stem or tail of the Y, i. e. the shaft or body of the rib, presents no sigmoid 

 flexure, but bends rather suddenly a little distad of the junction of the capitulum and 

 tuberculum. Through this bending, the rib may be said to have a sort of " angle." 



The capitulum is actually longer than that of the third rib, and relatively longer as 

 compared with the tuberculum, though not as compared with the whole rib. It is 

 actually the slenderest part. 



The head itself (fig. 75, iv', h) presents a convex articular surface, which is pre- 

 ponderatingly developed dorso-ventrally, being nearly twice as long in this direction as 

 transversely. 



The vertebral margin (between the capitulum and tuberculum) is less sharply concave, 

 its margins forming an angle of about 70°. The pneumatic foramen (/) is actually 

 larger, but relatively smaller. 



The tuberculum and its articular surface for the diapophysis {t) are much more 

 extended antero-posteriorly, the latter being thus twice as long as broad. 



The preaxial margin of the body of the rib is sharper than in the third rib ; in fact 

 a rounded ridge near the preaxial margin (on the visceral surface of the rib) seems to 

 answer to the actual ventral margin of the third rib, as, like the latter, it continues on 

 the prominence of the capitulum. If so, then the actual preaxial margin of the fourth 

 rib, at its proximal part, would seem to be formed from a lamella of bone being 

 developed on the ventral side of what was, in the third rib, the actual preaxial margin. 



The whole preaxial margin is nearly on one level vertically. 

 VOL. Tin. — PAKT VII. March, 1874. 3 q 



