1S79.] ANATOMY OF THE HOATZIN. 1 13 



Strong vinculum downwards to that of the flexor digitorum pro- 

 fundus before it runs on to supply the hallux itself. The determi- 

 iiation 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 

 fiir as the lower margin of the sternum ; and there is a small third 

 pectoral covered by it, as in all Gallinae, 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 Cracidse ; but it is present in the closely allied 

 Megapodidse, 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 

 vinculum place its non-passerine nature beyond a doubt. Addii);' 

 the tufted oil-gland and the inch-long colic cseca, the bird could only 

 be related to the Tinamidae, Gallinse, or Rallidse, from which it will 

 be remembered the Cuculidse differ in that they lack the oil-gland 

 tuft, and the Musophagidae in that they have no colic caeca. Opis- 

 thocomus, being holorhinaP, can have nothing to do with the Chara- 

 driiform birds. In the Rallidse there is only a single posterior 

 notch on each side of the carina sterni, at the same time that a crop 

 is never developed. These features, when correlated with the pecu- 

 liarities of the palate, remove them from the necessity of further 

 consideration. 



Opisthocomus must therefore, from what has been just shown, be 

 a Gallinaceous bird, or form a group by itself. As there is no 

 Gallinaceous bird without a direct articulation between the pterygoid 

 bones and tlie basisphenoidal rostrum, it is hardly possible to incfude 

 the Hoatzin along with them ; and yet it resembles them most 

 closely, as it does the Cuculidee, in thelength of its colic cseca and 

 the number of its rectrices. It is not far removed from the Muso- 

 phagidae as well. All these facts can be expressed as follows : — 



' Vide-p.Z.8ASro, p. 341. 



^ Method! Naturalis Avium disponendarum Tentamen. Stockholm, IS73, 

 p. 156. 

 ' Vide P.Z. S. 1876, pp. 195, 199. 

 * r. Z. S. 1873, p. 33. 



Pr.oc. ZooL. Soc— 187!), No. VIII. 



