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Show 1886.] GEOCOCCYX CALIFORNIANUS. 477 wards and forwards to the inner surface of the head of the tibia, where it becomes inserted, the point of insertion being found above that of the semimembranosus muscle, the insertional tendon of which overlaps it. The semimembranosus (Plate XLIV. figs. 1, 2; Plate X L V . fig. 1, Sm) in Geococcyx, thougb thoroughly developed, is rather a slender and thin muscle, markedly so when we compare it with the massive semitendinosus which overlies it. It arises from the outer surface of the ischium, for its posterior two thirds, on a line situated a few millimetres above the lower free edge of that element of the pelvis. The fibres gradually converge as they pass downwards and forwards, to terminate in a \ery delicate and thin ribbon-like tendon, which, passing between the broad tibial head of the gastrocnemius and the proximal extremity of the shaft of the tibia, becomes finally thereupon inserted on its internal surface. The hinder margin of the semimembranosus is free, while its border anteriorly is juxtaposed to the posterior edge of the adductors. In the bird before us the ambiens muscle (Plate XLV. fig. 1, amb) is conspicuously developed. It arises from the apex of the prominent prepubic spine of the pelvis, and the fibres passing directly down to the inner side of the femur, and parallel with that bone, form a strong fusiform muscle. As it approaches the patella it terminates in a small flattened tendon, which, piercing the fascial envelop of the knee-joint below the inferior apex of that sesamoid, passes round the joint, to become finally lost to the outer side and opposite the summit of the tibia, where some of its tendinous fibres merge with the fibres of origin of the flexor perforatus digitorum, or, at least, with one of its divisions. The ambiens is overlaid by the sartorius muscle, and in the figure is brought into view only through the aid of a small dissecting-hook and chain, which pull it forwards in order that it may be better seen. The femoro-caudal muscle and the accessory femoro-caudal are both present and fully developed. The femoro-caudal (Plate X L V . fig. I, fie) arises, tendinous, from the lower posterior border of the pygostyle. It soon becomes fleshy and as a narrow, muscular ribbon passes through the tissues overlying the lateral group of caudal muscles proper. Opposite the posterior border of the pelvis it expands to form a prettily-shaped and compressed spindle, closely covering the obturator externus muscle and the side of that bone. As it nears the femur it again contracts, receives the fibres of its accessory head, and is finally inserted upon the femoral shaft, at the posterior aspect of its proximal third. The accessory femoro-caudal (Plate X L V . fig. 1, a.f.c) arises beneath the overarching part of the postacetabular portion of the ilium, just behind the acetabulum and beyond. Its fibres pass obliquely downwards and forwards to join with those of the femoro-caudal, and to become inserted with them into the upper part of the femur as already described. The obturator externus (Plate X L V . fig. 1, o.e) arises from the outer surface of the ischium above the "obturator space," the |
