18 



MB. r. E- BEDDARD ON THE 



[Jan. 15, 



There is no need to enter into comparisons between this and 

 other parts of the shoulder-girdle in the Ground-Hornbills and the 

 arboreal forms, since the structure and relations are as nearly as 

 possible identical. This seems to show that use is a more important 

 factor than disuse in the modification of organs, since the hind 

 limbs show noteworthy differences. 



Skull.— Yery shght, but still perfectly recoguizable and defin- 

 able, differences distinguish the skulls of the two species of 

 i?i«co;w<s (c/'. text-figs. 2, 3). 



The most striliing difference is, however, possibly a sexual one : in 

 B. cafer the bony prominence on which sits the casque of the bird 

 is much lower than it is in B. abyssinkus, and at the same tiuie its 

 texture is decidedly more solid ; in B. abyssinicus this part of the 

 skull is formed of very delicate cancellated bony tissue Avhich 

 immediately underlies the horny casque. My specimen of B. cafer. 



Text-fig. 2 



Skull of Bucormis abysdnicus, (^ . ( X -|.) 



however, is a female bird; the skeleton of B. ahijsslnicus belongs 

 to a male. 



When the two sliulls are viewed from above, they can be readily 

 distinguished by the gi-eater breadth of that of B. cafer. The 

 widest part is just behind the orbits. The measurements in the 

 two species are as follows : 



Bucorvus cafer Length 206 mm. ; breadth 63 mm. 



B. abijssinicus Length 203 mm. ; breadth 59 mm. 



A very small fragment of the tip of the beak in B. ahyssinicus 

 was, however, broken off and lost. This would therefore 'increase 

 the length of the skull in that species, and thus render the propor- 

 tions a little more striking than is apparent from the measurements. 

 The greater breadth of the skull in B. cafer can, however, be veil 

 appreciated without any measurements at all. 



A third feature in which the skulls of the two Ground-Hornbills 



