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Show 1886.] GEOCOCCYX CALIFORNIANUS. 473 responding side. The fibres, forming an oblong and rather thick muscle, pass downwards and backwards to the region in front of the knee. Here it becomes inserted by a special slip of fascia that is thrown off and merges with the general fascia surrounding the knee-joint ; and, secondly, by a more carneous insertion into the inner half of the superior rim of the cnemial crest of the tibia and the continuous inner margin of the summit of that bone. The sartorius bounds anteriorly the superficial group of muscles of the thigh ; consequently its anterior border is free. Its posterior border above unites quite intimately with the overlapping gluteus primus muscle ; while this border below is sharp and free, although here, too, the gluteus also overlaps it, and a delicate connective tissue binds them together. The gluteus primus (Plate X L I V . fig. \,gl.pr) constitutes that great and rather complex muscle which makes up the central fleshy portion of the outer aspect of the thigh. It arises by a strong fascia from the summit of the coossified neural spines of the anterior sacral vertebrae, and by carneous fibres from the outer rim and under surface of the whorl-like, overarching portion of the ilium behind; and finally from the contiguous portion of the pelvis over the anti-trochanter, between these anterior and posterior origins. In front the muscle consists first of a strong layer of semitendinous fascia, which closely overlies the gluteus medius muscle beneath it, and overlaps the sartorius anteriorly. The posterior origin and mid-division become rapidly carneous and more massive as we proceed in the direction of the caudal extremity of the body. So that, where we find it arising from beneath the overarching part of the ilium behind, the muscle fills about one fourth of the convexity there formed, the semitendinosus filling the remainder of this curious cavity. The fibres of the strong, semitendinous, muscular sheet springing from these several origins, or rather along this continuous line of origin, now pass, converging as they do so, towards the anterior aspect of the knee-joint. The semitendinous portion anteriorly becomes fleshy as it arrives along the outer pelvic margin, with which it is quite intimately connected. The hinder division of the muscle remains thick and carneous until it comes to the knee-joint. Here all the fibres again become tendinous and fascia-like, and, uniting with a similar structure contributed by the extensor femoris lying beneath it, the combined sheath thus formed surrounding the well-developed patella, closely invests the front and sides of the knee-joint, and is finally inserted all round the anterior and externo-lateral borders of the summit of the tibia. The most superficial muscles of the leg are the gastrocnemius and the peroneus longus. As we would naturally expect, the gastrocnemius muscle (Plate X L I V . fig. 1, g) in Geococcyx is wonderfully well-developed. All three of its heads are strongly defined, and the fleshy belly of the muscle is massive and thick. Its external head arises, curiously enough, by two perfectly distinct tendinous slips. One of these, a strong, flat tendon, comes off from |