TUBINAEE8 



449 



•structure. In Garrodia the first three bronchial rings are 

 complete. In Thalassoeca glacialoides the last four tracheal 

 rings are incomplete behind, and are quite Hke the four 

 succeeding bronchial semi-rings, being, moreover, like them. 



Fig. 211. — Skdll of Diomedea exulans 



(after Huxley). 

 Fo, vomer ; Mxp, maxillo palatines ; Pt, pterygoid. 



Fig. 212. — Skull of Procellaria 

 gigantea (afteb Huxley). 



Pl^ palatines ; *, basipterygoid process. 

 Other letters as la fig. 211. 



ossified. This modification is carried to its furthest extreme 

 in Ossifraga. In this petrel the last nine or ten tracheal 

 rings are incomplete in front, and the last twelve or so are 

 incomplete behind. Thus the membrana tympaniformis 

 does not commence for some distance away from the bifur- 

 cation of the respiratory tube, the rings being double and 



G a 



