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 flewor. With the second 
and third toes, however, the carneous fibres of the muscle under 
consideration are continued all the way to the trochleze, 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 Geococcyw 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. 1, ¢ib. post) is a very slender 
muscle in Geococcyz, 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 ¢idialis 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 flewor 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 flewor perforatus indicis secundus pedis (Plate XLIV. fig. 2, 
J p,) is even a better developed muscle than I found it to be among 
typical Corvidee, 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. Zoot. Soc.—1886, No. XXXII. 32 
