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Show 1886.] GEOCOCCYX CALIFORNIANUS. 481 This tendon passes round beneath the trochlea for the fourth toe and is really inserted on the underside of the basal joint of this digit at its proximal extremity ; so that in the case of this toe it seems as though it would act almost as a flexor. With the second and third toes, however, the carneous fibres of the muscle under consideration are continued all the way to the trochleae, where they terminate, in either case, in a strong, flat tendon, which passing over the joint is inserted on the upperside of the proximal extremity of the basal joint. Here, of course, the muscle acts (in the case of the second and third toes) as an auxiliary to the long extensor. Not a little room is here open to us for speculation as to how the tendon of this short extensor in the case of this fourth toe exactly came to assume its present point for insertion, as the digit gradually and finally became permanently reversed. Indeed, the high development of this short extensor in Geococcyx over the vast majority of the class is, too, an interesting fact; and did the reversion of the digit precede or follow the muscular development? No doubt the completeness of the latter, and its perfection for an avian type, has come about as a demand on the part of the habits of the bird itself and its marvellous fleetness of foot. The tibialis posticus (Plate X L V . fig. 1, tib.post) is a very slender muscle in Geococcyx, but closely resembles the same muscle as I have found it in all other birds which I have examined for their myology. M y reasons for terming it the tibialis posticus are fully given in m y M S S . and will appear in due time. It seems to be one of the peronei of the senior Edwards. As in a number of the Passeres, we find it here to arise from the antero-lateral aspect of the shaft of the fibula below the tubercle for the insertion of the biceps flexor cruris, from the interosseous membrane between the leg-bones, from the contiguous surface of the shaft of the tibia, and, finally, from the fascia separating it from the deep flexors of the leg. The fibres pass directly down the outer side of the tibia as a long, slender, fusiform muscle. At the lower fourth of the shaft of this bone they terminate in a small tendon, which, passing in front of the external malleolus, crosses the ankle-joint to become inserted into the supero-external rim of the summit of the tarso-metatarsus. The flexor perforatus indicis secundus pedis (Plate X L I V . fig. 2, f.pj is even a better developed muscle than I found it to be among typical Corvidae, some of which I have recently dissected, and it is fully as well individualized. It arises from the fascia at the outer side of the knee-joint, and from the contiguous surface of the external condyle of the femur. Here it receives the anastomosing fibres of the extremity of the tendon of the ambiens. The muscle is fusiform in shape and accurately moulded on the flexor it covers at its side. Its tendon in descending the leg is thin and ribbon-like. At the ankle it passes through the tibial cartilage, and crossing the joint goes through, with the second tier of tendons, the cartilaginous cap on the back of the hypotarsus of the tarso-metatarsus. Passing down behind this latter bone, and PROC. ZOOL. Soc-1886, No. XXXII. 32 |