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 develop- 

 ment 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 XLV. fig. I, 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. 

 My reasons for terming it the tibialis posticus are fully given in my 

 MSS. 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 indlcis secundus pedis (Plate XLIV. fig. 2, 

 f.p^ is even a better developed muscle than I found it to be among 

 typical Corvidse, 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 



