OSTEOLOGY 121 



in its relations to other structural similarities and dissimilari- 

 ties that the fact is not of great use in classification. 



The same remark may be made about the foramen cora- 

 coideum perhaps, which, again, is found in many birds and 

 absent from others. As to the morphological significance of 

 this foramen, which transmits a nerve twig to the pectorahs 

 secundus, it may perhaps be regarded as the boundary 

 between the coracoid and the procoracoid. 



In the course of the development of the common fowl, 

 according to Miss Lindsay, whose figures are here reproduced, 

 there is a very considerable trace of the procoracoid. The 

 three elements of the shoulder girdle are perfectly distinct 

 from each other in the young embryo, but become fused (the 

 scapula and the coracoid), again to get separate in the older 

 chick. This temporary fusion may be significant of the 

 struthionic condition to be described later. The intermediate 

 piece is, it will be noticed, triangular in form, the elongated 

 aspect of the adult cotacoid being acquired later. Miss 

 Lindsay is of opinion that this change of form is to be cor- 

 related v?ith the disappearance of the anterior section of the 

 bone, as indicated in the accompanying diagram, the disap- 

 pearing (shaded) part being the equivalent of the procora- 

 coid. 



The interrelationship of the scapula to the coracoid offers 

 facts of some importance. In the ostrich tribe the two bones 

 are firmly ankylosed ; this is not the case with the young, 

 but it is plainly the case with the adult. In carinate birds, 

 on the other hand, there is not ankylosis, but a close union 

 by means of fibro-cartilage. It appears, however, that in 

 Didus (exceptionally ?) there is an actual synostosis, which of 

 course bears out the suggestion that the synostosis of the 

 ratite birds has something to do with their loss of the power 

 of flight. In the ratite birds and in Hesperornis the scapula 

 and the coracoid are nearly in the same straight line, the 

 angle in Apteryx varying from 150° to 122°, whereas in the 

 carinates the two bones are at right angles or at an even 

 acute angle. That this is not a morphological distinction, 

 but is distinctly related to the development of the shoulder 



