1896.] ANATOMY OF THE HOATZIN. 621 
Opisthocomus, Fowls, and Pigeons, and Garrod’s’ suggested relation- 
ship with Fowls and Cuckoos, is borne out. But the Gallinaceous 
birds are more primitive in the character of their mid-gut, and from 
this point of view Opzsthocomus must be regarded as less primitive 
than them, while both Huxley and Garrod from other considerations 
regard it as more primitive. 
The subsidiary looping and consequent length of the rectum or 
large intestine between the insertion of the ceca and the cloaca is 
a striking feature found only in few birds, all of which have the 
intestines otherwise primitive : it reaches a maximum in the Ostrich, 
giving the intestine of that bird a curiously mammalian aspect ; 
it is absent in Casuarius, Dromeus, Apteryw, and Rhynchotus ; it 
is well-marked in Rhea, Chauna, Palamedea, and in Opisthocomus. 
I am unable to correlate it with any degree of development of the 
cca or with habits or food. 
Muscles of the Visceral Skeletal Apparatus. 
Although many papers have been written which include myo- 
logical descriptions of Opisthocomus, I can find no account of the 
muscles of the jaws and hyoid. Ina large number of birds the 
hyoid muscles in particular are difficult to isolate and dissect ; 
many of them are extremely delicate, and the fasciz of adjacent 
muscles blend with each other at many points. In Opisthocomus 
these muscles are particularly stout and free from each other; on 
removal of the skin covering the space between the mandibles they 
may be dissected out (see fig. 2, p. 622) with great ease. 
Mylohyoid antertor.—This pair of muscles forms a broad trans- 
verse band stretching between the inner edges of the rami of the 
mandible. The fibres from the opposite sides pass straight across, 
not meeting in a median raphe as occurs in Chauna and the Goose ; 
but the muscle is not, as in Paulamedea and the Goose, divided into 
an anterior and posterior portion. It is much stouter than ina 
typical Pheasant like Lophophorus impeyanus. 
Mandibular Glands.—Behind the symphysis, and with their 
proximal border just covered by the mylohyoid anterior, lie a pair 
of large ovoid glands, opening, as in the similar glands of Chauna’, 
by a number of small apertures into the floor of the mouth, where 
the mucous membrane reaches the horny edge of the lower jaw. 
In the Pheasant (Lophophorus) these glands are very large and 
lobulated. 
Mylohyoid posterior —This, as in all birds that I have examined, 
or of which I can find record on the point, is a large muscle dividing 
almost immediately into an anterior deeper layer and a posterior 
more superficial layer. In Opisthocomus there is a large common 
origin from the outer side of the ramus of the jaw, immediately 
anterior to the insertion of the depressor muscle. From this 
comes the whole of the posterior, more superficial division of the 
1 “On the Anatomy of the Hoatzin,” P. Z.S. 1879, p. 109. 
2 « Anatomy of Chauna chavaria,” P. Z, 8. 1895, p. 350. 
