24 ON THE ANA.TOMY Or PICABIAN BIRDS. [Jan. 15, 



A final point of some little interest concerns the bones of the 

 foot. Perching and walking on the ground are clearly two very 

 different modes of using the feet, and we should therefore expect 

 to find some corresponding differences in the structure of the foot. 

 As a matter of fact, such differences do occur in the two series of 

 Horubills. In Buceros the middle metacarpal is if anything slightly 

 longer than that of the second toe, while the fourth metacarpal 

 is about one half of the length of the two metatarsals of the 

 middle toes. In Bucorvus the second metatarsal is slightly 

 lono-er, and also rather stouter, than the third metatarsal, while 

 the*^ fourth metatarsal is not so much reduced as in Buceros. 

 It is clear then, that, apart from the differences in length which 

 distinguish the genera, the prevailing toe in the bipedal Bucorvus 

 is the second, which is really functionally the first toe, for the true 

 first toe is of course turned backwards. It is true that the third 

 toe is tlie longest ; but nevertheless the increased length of the 

 second metatarsal gives to that toe a preponderance in the foot. 

 This state of affairs contrasts with that observable in the quadru- 

 pedal Ungulates, where it is the middle toe (or the two middle toes) 

 that is the prevailing one. In correspondence with the greater 

 length of the second metatarsal, the tibio-tarsus is more strongly 

 developed on that side and projects beyond the rest of the bone, 

 the articular surface of which is therefore oblique to the trans- 

 verse axis of the leg. In BucerosVae line of articulation is exactly 

 transverse. This will be apparent from the drawings exhibited 

 (text-figs. 4 & 6, pp. 22 & 23). The last-mentioned feature is not, 

 however, distinctive of Bucorvus: for in Bhytidiceros the same 

 obliquity at the end of the tibio-tarsus occurs. In Toccus, more- 

 over, in addition to the obliquity, the second metatarsal is the 

 longest. 



From this description of certain features in the anatomy of 

 Bucorvus, the osteological characters of the genus and of the t\\'o 

 subspecies B. cafer and B. ahyssinkus can be formulated : — 



Genus Bucoevus.— Cervical vertebrae 13, short and broad, with 

 concave centra and transverse processes forming a gutter 

 beneath. No catapophysial canal or approach towards one. 

 Pygostyle comparatively rudimentary. 



Skull flat above, with marked shelf-like supraorbital plates. 

 Toramen magnum ventral in position. Pterygoids straight. 

 Basipterygoid processes rudimentary. Bony core of casque 

 not sharply marked off from maxilla in front. 



Second metatarsal the stoutest and longest : end of tibio- 

 tarsus oblique. Tibia twice as long as femur; tibio-tarsus 

 one and a half times as long. 



B. c«/er.— Neck comparatively short. Sternum rather deeply 

 notched with one incision. Skull broad in proportion to 

 length. 



B. ahijssinicus — Neck comparatively long. Sternum not deeply 

 iiotched. Skull narrower in proportion to length. 



