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Show 24 ON THE ANATOMY OF PICARIAN BIRDS. [Jail. 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 Hornbills. 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 longer, 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 the 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 quadrupedal 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 aud projects beyond the rest of the bone, the articular surface of which is therefore oblique to the transverse axis of the leg. In Buceros the line of articulation is exactly transverse. This will be apparent from the drawings exhibited (text-figs. 4 & 5, pp. 22&23). The last-mentioned feature is not, however, distinctive of Bucorvus: for in Rhytidiceros the same obliquity at the end of the tibio-tarsus occurs. In Toccus, moreover, 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 two subspecies B. cafer and B. abyssinicus can be formulated : - Genus BUCORVUS.-Cervical vertebra? 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. Foramen 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. cafer.-Neck comparatively short. Sternum rather deeplv notched with one incision. Skull broad in proportion to length. B. abyssinicus.-Neck comparatively long. Sternum not deeply notched. Skull narrower in proportion to length. |