ON THE GENUS PTILOPUS. 347 



'' In Megacephalon maleo^ on the hinder part of the head a bald region 

 is to be found, which rises chiefly behind ; it is covered with a strong black 

 epithelium. In the young bird this naked protuberance is not yet developed ; 

 . . . the head is covered all over with feathers. . . . 



" The Casuaries, as to the protuberances on the head, approach closely 

 the Gallinaceous birds." 



Page 41 : — 



'' Summary and Conclusion, 



'^The results arrived at may be summarized under the foUow^ing 

 heads : — 



" (1) The bony protuberances on the bills and skulls of birds 

 are either swellings full of bladders, or spongy metamorphoses of certain, 

 often very difi^erent bones. 



" (2) They are always pneumatic ; that is to say, the air enters from 

 the lungs or directly from the nose. 



" (3) These protuberances either occur in both sexes or only in one, 

 and in the latter case in the male. They must generally be understood as 

 results of sexual selection. 



" (4) Where these protuberances occur in both sexes in the same 

 manner, they are very old (^Numida, Casuarius, most species of Buceros^ 

 &c.), and consequently are already developed in the very young birds. 

 Where they only occur in one sex they are acquired much later in the series 

 of proo:enitors ; they consequently are developed only at the commencement 

 of puberty (^Fulica nigra &c.). 



" (5) In certain families single members possess bony protuberances 

 (^Fuligula nigra, Craw pauxiy Numida meleagris, &c.), which are represented in 

 others by similar phenomena, but which belong to another system of the body 

 (Cygnus olor, Crax alberti, Numida plumifera, N. cristata, &c.). Between these 

 two forms are connecting links (^Fuligula spectabilis, Crax mitra}. 



