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 XLV. fig. 1, 

 Sm) in Geococajx, though 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 belovif the 

 inferior apex of that sesamoid, passes round the joint, to become 

 finally lost to the outer side and opposite the sumuiit of the tibia, 

 where some of its tendinous fibres merge with the fibres of origin of 

 t\\e flexor perforatus diyitorum, 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. 



"i^ie femoro-caudal (Plate XLV. fig. l,/.c) arises, tendinous, from 

 the lower posterior border of the pygostyle. It soon becomes fleshy 

 and as a narrow, muscular ribbon passes through the tissues over- 

 lying 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 prox- 

 imal third. 



The accessory femoro-mudal (Plate XLV. fig. 1, a.f.c) arises be- 

 neath the overarching part of tlie 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, 

 aud to become inserted with them into the upper part of the femur 

 as already described. 



The obturator externus (Plate XLV. fig. 1, o.e) arises from the 

 outer surface of the ischium above the "obturator space," the 



