500 ARKIV FÖR ZOOLOGI. BAND 1. 



Both plates are defined from each other by a longitudinal 

 groove all along the side of the mandibular ramus. In the 

 Lamellirostres generally rostrale and mentale constitnte the 

 main working parts of the bill when they seize anything 

 with this organ. The plates mentioned are therefore härd 

 and horny while the others have become more or less softened 

 to improve the sensibility of the bill. There is nevertheless 

 no difficulty in discerning the different elements mentioned 

 above, although their consistence at least partly approach 

 that of a cere. 



A long bill wonld not be suitable for a bird of prey. 

 The Falconiformes have therefore got their bill reduced in 

 length at the same time as its shape was modified. This 

 reduction is most prominent in the most specialised Falcones. 

 In the still living, more generalised types of the diurnal birds 

 of prey, as for instance the Vultures and others, the bill has 

 retained a certain length, but in such 

 cases, only the tip is provided with a 

 strong and quite härd horny armature 

 while the base is covered by a softer 

 layer, a cere. This is, however, in Ca- 

 thartidce and Vulturidce still more or less 

 horny, but in the more typical and specia- 

 lised Falconiformes it is thicker and 

 Fig. 10. The bill of a ju -, , , , -, ., n 



young Pernis. sorter and better deservmg its name ot 



cere. An examination of for instance a 

 Cathartes or a Neophron, a Helotarsus and a Falco is very 

 instructive as showirig the gradual transformation of the bill. 

 I venture now to put up the theory already alluded to above 

 and which shall be strengthened by proofs from other groups 

 as well, that the cere represents the reduced horny 

 shields of the basal rhamphotheca. It ought then to 

 be made out by which parts the strong and härd rhampho- 

 theca of a bird of prey is formed. A look at a downy young 

 of a member of Falconidce reveals this at once especially if 

 a comparison with a young of Cancroma is made. It is then 

 apparent that in the former the »Anlage» of the hook of the 

 upper bill which is just developing and hardening corresponds 

 to the rostrale of the latter and in the same way on the 

 lower jaw there is a simple mentale which in both has the 

 same situation and seems quite unmistakable. I mean thus 



