E. LÖNNBERG, ON THE RHAMPHOTHECA OF BIRDS. 483 



draw the conclusion as to the natural affinity of the diffe- 

 rent groups of birds which might be made on the basis of 

 this theory. 



As Jon g as the ancestral birds were provided with teeth the 

 horny covering of the jaws was of comparatively less importance, 

 but with the loss of the teeth the condition was changed. The 

 prcemaxillare grew in size and strength while the formerly 

 toothed maxillare became reduced and weakened. It is pre- 

 sumable that simultaneously with the growth of the prce- 

 maxillare its horny covering also must be more developed 

 and gain in strength. Thns the enlarged rostrale was deve- 

 loped in such a way that it could assume new functions 

 and from a merely shielding organ become a grasping organ, 

 snbstitnting the löst teeth in the act of catching and holding 

 the prey and so on. The mentale was developed in a corres- 

 ponding manner. With the increased development of the 

 premaxillary x and rednction of the maxillary tract it is 

 probable that the anterior labials were enlarged and the 

 posterior more or less reduced. 



This is a short sketch indicating how I think the deve- 

 lopment has proceeded and it remains now to be seen whether 

 any morphological facts may be gathered from the recent 

 now living birds which can support this theory. 



If then the Struthiones are considered in the first rank 

 the following is revealed. (By using the word Struthiones 

 I have not expressed any opinion as to the monophyletic 

 origin of the birds usually termed so, but the name is only 

 used for the sake of convenience.) 



In Bhea americana I find at the end of the lower jaw 

 a large unpaired piece occupying the position of the mentale 

 of the reptiles and according to my opinion homologous to 

 the same. Either side of the lawer jaw is covered by a 

 longitudinal horny shield correspondiug in a similar way to 

 one or several fused infralabials in the reptilian series. 

 The upper jaw bears at its distal end a broad median piece 

 which corresponds to the rostrale of the reptiles in its 

 situation and is to my mind homologous to the same. The 



1 The prolongation of the premaxillary region in the birds when compared 

 with the reptiles makes itself known when the situation of the nostrils is 

 considered, at or near the base of the bill in the former (with few exceptions) 

 but at the end of the snout in the latter. 



