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Show 18 MR. F. 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-Hornbillsand 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,-Very slight, but still perfectly recognizable and definable, differences distinguish the skulls of the two species of Bucorvus (cf. text-tigs. 2, 3). The most striking 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. abyssinicus, and at the same time its texture is decidedly more solid ; in B. abyssinicus this part of the skull is formed of very delicate cancellated bony tissue which immediately underlies the horny casque. M y specimen of B. cafer, Text-fig. 2. Skull of Bucorvus abyssinicus, o*. (X{.) however, is a female bird; the skeleton of B. abyssinicus belongs to a male. When the two skulls are viewed from above, they can be readily distinguished by the greater 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 m m . ; breadth 63 m m. B. abyssinicus Length 203 m m . ; breadth 59 m m. A very small fragment of the tip of the beak in B. abyssinicus was, however, broken off and lost. This would therefore increase the length of the skull in that species, and thus render the proportions a little more striking than is apparent from the measurements. The greater breadth of the skull in B. cafer can, however be well appreciated without any measurements at all. A third feature in which the skulls of the two Ground-Hornbills |