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Show 1896.] ANATOMY OF THE HOATZIN. 621 Opisthocomus, Fowls, and Pigeons, and Garrod's1 suggested relationship 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 Opisthocomus 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 caeca 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, Bromceus, Aptergx, and Bhgnchotus; it is well-marked in Rhea, Chauna, Palamedea, and in Opisthocomus. I am unable to correlate it with any degree of development of the caeca 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. In a large number of birds the hyoid muscles in particular are difficult to isolate and dissect; many of them are extremely delicate, and the fasciae 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 anterior.-This pair of muscles forms a broad transverse 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 in the similar glands of Chauna2, by a number of small apertures into tbe 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 chavarui," P. Z. S. 1895, p. 350. |
