AXIAL SKELETON OF THE OSTRICH. 



445 



its sternal end, and only a little more than half the length of the second sternal rib. 

 It is articulated distally in front of the fossa, which is preaxial to the first vertical 

 septum of the pleurosteon. 



It only seems to be connected with the third vertebral rib by soft tissues intervening 

 between them. 



STERNAL RIBS OF RIGHT SIDE (i-vi, | natural size ; i', the first rib of an immature specimen, 



natural size; ii', iii', iv', v', and ti', | natural size). 



Fi?. 76. 



i-vi. Lateral aspect of the six sternal ribs ; u'-vi'. The articular surfaces of the second, third, 

 fourth, fifth, and sixth sternal ribs. 

 In aU the figures s represents the concavity which articulates with the dorsal convexity of each pleurosteal 

 septum, and i represents the concave articular surface for the ventral articular convexity of each such 

 septum. 



The Second Sternal Rib. 



The second sternal rib (fig. 76, ii & ii') is of about the same length as the second 

 vertebral rib, though it articulates with the fourth vertebral rib. It is irregularly 

 rounded at its proximal end ; but its distal end is very greatly expanded dorso-ventrally, 

 and compressed pre- and postaxilly, with a pneumatic foramen on its postaxial surface 

 at its expanded part. 



The ventral end of the rib presents two superimposed elongated concavities (s & i) to 

 fit on to the two superimposed articular convexities of the first septum of the pleurosteon 

 of the sternum (fig. 79, i, s). The bone is slightly curved, with the convexity dorsad. 



The Third Sternal Bib. 

 This rib (fig. 76, in & iii') is rather less than half as long again as the second sternal 

 rib, and is more curved. 



