712 MISS B. LINDSAY ON THE AVIAN STERNUM. [June 1 6, 



posterior-lateral process is well marked ; witness the sternum in the 

 Hesperornis, Dinornis, Dodo, Apteryx, Solitaire, and Diduncuhis 

 (cf. fig. v.). Other noteworthy instances of a single posterior- 

 lateral" process, probably of primitive type, are given in Tinamus, 

 Herodias, Spheniscus, Aptenodytes, and in most famihes of the 

 Passeres, in which latter the shape of the process shows a tendency 

 towards the filling up of the sternum. 



Fig. V. 



Archaic types, with posterior-lateral processes. 



1, Sternum of Bidus incjiivs ; 2, of Dinornis cras^us, from N. Otago, New Zea- 

 land ; 3, oi Dinornis, from Hector Eange, New Zealand ; 4, of Pezophaps 

 soUtaria ; 5, of Aptcryx oweni ; (i, of A. mantcUi. (All C.C, reduced.) 



r, is a fontanel imperfectly closed. 



Finally, it is to be remarked, if the reference to a mechanical 

 theory of the origin of the posterior processes seems to require 

 apology, that the very existence of the sternum may perhaps be 

 explained by a mechanical theory. The ends of the ribs fuse, as 

 Rathke showed, into a continuous band. Now it is not improbable 

 that this is primarily due to the establi:^hment of lungs. The differ- 

 ence between the capacity of the thorax in inspiration and expira- 

 tion necessitates that the extra length of each rib shall be packed 

 in a loop during the latter position {cf. fig. VI., 2). Now in the 

 embryo, the thorax being in the position of rest, these loops lie close 

 together. Thus it is easy to see how the original fusion of rib-ends 

 may have taken place. 



