1896.] ANATOMY or THE HOATZIN, 621 



Opislhoeomus, Fowls, and Pigeons, and Garrod's ' suggested relation- 

 ship with Fowls and Ciiclcoos, is borne out. But the Gallinaceous 

 birds are more primitive in the character of their mid-gut, and from 

 this point of view Opisthoeomus must be regarded as less primitive 

 than them, while both Huxley and Garrod from other considerations 

 I'egard it as more primitive. 



The subsidiary looping and consequent length of the rectum or 

 large intestine between the insertion of the coeca 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, Dromceus, Aptery.v, and Jihynchotus ; it 

 is well-marked in lUiea, Ghauna, Palamedea, and in Opisthoeomus. 

 I am unable to correlate it with any degree of development of the 

 ca3ca or with habits or food. 



Muscles of the Visceral Sheletal Apparatiis. 



Although many papers have been written which include myo- 

 logical descriptions of Ojiisthocomus, I can find no account of the 

 muscles of the jaws and hyoid. In a large number of birds the 

 hyoid muscles in particular are diflluult to isolate and dissect; 

 many of them are extremely delicate, and the fascife of adjacent 

 muscles blend with each other at many points. In Opisthoeomus 

 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 anterior. — This pair of muscles forms a broad trans- 

 verso 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 Palamedea and the Goose, divided into 

 an anterior and posterior portion. It is much stouter than in a 

 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 iu the similar glands of Ghauna", 

 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 Opisthoeomus 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 

 oomes the whole of the posterior, more superficial division of the 



> •■ On the Anatomy of the Hoatzin," P. Z. S. 1879, p. 109. 

 ^ " Anatomy of Chauna chavaria" P. Z. S. 1895, p. 350. 



