18S5.] MISS B. LINDSAY ON THE AVIAN STERNUM. 603 



of he lib?, i. e. is not, like that of the muscle-plate?, intravertebral. 

 Ths supercostal position it will be seen, however, from coin- 

 j)arison with other types, is only a secondary arrangement occurring 

 during part of their course, in consequence of the curvature of the ribs. 

 Thfy represent, in fact, traces of the intercostales externi of ribs in 

 their primitive arrangement, before they acquired their present 

 modifications in connection with the sternum. 



4. Although the sternum as a whole is greatly lengthened in the 

 course of its development, owing to the appearance of the metasternum, 

 yet its costal portion loses one posterior attached rib ; tiiere are six 

 sternal ribs in the embryo, five in the adult. Further, a seventh rib, 

 although never attached to the sternum, shortens considerably in 

 the course of development, which implies that it was once a sternal 

 rib. The position of the primitive intercostales externi of the two 

 unattached posterior ribs in the four days' embryo, being, as they 

 are, longer than the ribs they accompany, leaves little doubt that both 

 originally entered into the formation of the primitive costal bands. 



5. The series of embryos shows the gradual addition of the 

 anterior lateral process to the costal sternum. 



6. The 7 days' embryo shows an anterior part which may pro- 

 bably be compared to the manuhrium slerni of mammals. Its 

 extent in front of the first sternal rib, at a stage wlien none of the 

 parts known to be of secondary origin have as yet been added to 

 the costal sternum, taken in connection with the existence of two 

 anterior spinal ribs of which the second is very long, seems sufficient 

 to prove its costal origin. Further, St. George Mivart ^ records the 

 appearance in the Ostrich of a rudimentary sternal rib, the first of 

 six, which was not attached to the corresponding spinal rib (the 3rd) ; 

 from the number and position of the spinal ribs he describes, namely 

 ten, whereof the two posterior are not attached to the sternum, it 

 appears that this rudiment corresponds with the first sternal rib, 

 fully attached to the corresponding spinal portion, of the seven 

 embryos here described This affords a further reason for supposing 

 that the process of atrophy extended in the above-named specimen 

 to the third spiual rib has taken place already in the case of the 

 first and second. 



7. Comparison of the 15 days' embryo with preceding stages 

 shows the addition of the metasternum to the costal sternum. 

 Although this region, seen in all subsequent stages of the embryo, 

 is in perfect continuity with the cartilage of the original costal 

 sternum, yet in the adult (as appeais in Plate XLIT. fig. 9) only a 

 small part of it is ever ossified. In Bhea still less of it is ossi6ed, 

 so that the halves of the sternum remain united by cartilage only. 

 Apparently these do not, however, correspond exactly with the 

 primitive halves ; for a fontanel, nearly closed by thinner bone, marks, 

 in some specimens, a boundary corresponding in position with the 

 notch that separates off the posterior lateral process of the Ostrich 

 (c/fig.IV./, p. 710). 



1 "Axial Skeleton of the Ostrich {Slnithio camehts)" by St. George Mi^art, 

 F.E.S. : Trans. Zool. Soc. x. p. 1. 



Proc. Zool. Soc— 1885, No. XLV. 45 



