Page 113. 
468 ON THE ANATOMY OF THE HOATZIN. 
completely covers the biceps cruris superficially. The fifth gluteus, 
which runs from the ilium a short distance behind the acetabulum, 
and covers with its triangular tendon the trochanter of the femur, is 
present, but small. The semitendinosus and its accessorius are both 
large, as are the femoro-caudal and its accessorius. The myological 
formula,* as far as these muscles are concerned, is therefore AB. XY. 
The ambiens muscle is present and small; but its slender tendon, in 
every case but one of the six knees I have examined, is lost upon the 
capsule of the front of the knee. In the one instance it traversed the 
fibrous tissues of the quadratus-tendon, as in other birds where it is 
present, to join the digital flexors in the back of the leg. A similar 
imperfection in the development of the ambiens is sometimes found 
in Sula bassana, Stringops habroptilus, and in the species of the genus 
CGidicnemus. The obturator internus is triangular in shape, as in the 
Galline. 
In the deep tendons of the foot, the flexor hallucis longus sends a 
strong vinculum downwards to that of the flexor digitorum profundus 
before it runs on to supply the hallux itself.t The determination of 
this point the late Prof. C. J. Sundevall much desired, as in the only 
specimen he had the opportunity of examining, and that imperfectly, 
the apparent absence of the vinculum favoured its Passerine affinities. 
As, however, is stated above, the vinculum is present and large in the 
individuals dissected by myself. 
In the upper limb, the great pectoral muscle is much reduced at 
its furcular and manubrial origins, over which the crop is placed. It 
is thicker lower down. The fibres of the second pectoral descend as 
far as the lower margin of the sternum; and there is a small third 
pectoral covered by it, as in all Gallinew, although in Opisthocomus 
it is reduced in size. The biceps humeri muscle sends a peculiarly 
large fasciculus to the tendon of the tensor patagii longus, which 
reaches it opposite the middle of the patagium.§ This slip I never 
find developed in the Cracide; but it is present in the closely allied 
Megapodide, and in all the other Gallinaceous birds. 
The above-mentioned myological facts throw some light on the 
affinities of Opisthocomus. The presence of two carotid arteries, an 
ambiens muscle, an accessory femoro-caudal, and a deep plantar vin- 
culum place its non-passerine nature beyond a doubt. Adding the 
tufted oil-gland and the inch-long colic ceca, the bird could only be 
* Vide “Proceedings of the Zoological Society,” 1874, p. 111. (Supra, p. 208.) 
+ Vide “ Proceedings of the Zoological Society,” 1875, p. 341. (Supra, p. 290.) 
t “Methodi Naturalis Avium disponendarum Tantamen.” Stockholm, 1873, 
p- 156. 
§ Vide “Proceedings of the Zoological Society,’ 1876, pp. 195, 199. (Supra, 
pp. 324, 329.) 
