MUSOPHAGI 285 



alliance between Musophagidae and Cuculidae it may be 

 useful to tabulate the principal divergences in the skull. 



Corythaix has 14 cervical vertebrm ; ' the atlas is notched, 

 not perforated by the odontoid process. Four ribs reach 

 the sternum, which is doubly notched and has a strong 

 spina externa. 



In the pelvis the prepubic process is very markedly 

 developed, as in Geococcyx. 



The clavicle comes into contact both with the scapula 

 and with the moderately large procoracoid ; the latter is 

 fused with the acrocoracoid,^ making thus a complete bridge 

 over the sulcus supracoracoideus. The coracoids slightly 

 overlap at their articulation with the sternum. 



The hsemapophyses are characteristic. In Corythaix albo- 

 cristata.Gll has paired processes ; on C12 and C13 the hsema- 

 pophyses form a continuous ventral keel to those vertebrae. 

 The three following vertebrae have hsemapophyses. The 

 extremities of those of Dl and D2 are expanded with a 

 median ridge, owing to the catapophyses having descended 

 them.' 



OPISTHOCOMI 



Definition. — Aftershaft present. Oil gland feathered. Kectrices, ten. 

 Quintooubital Muscle formula, ABXT + . Biceps slip and 

 expansor secundariorum present. Carotids, two. CsBca present. 

 Skull holorhinal, sohizognatlious, without basipterygoid pro- 

 cesses. Sternum peculiar in form, wider behind than in front ; 

 the spina externa ankylosed with furcula. 



There is no doubt that this group, consisting of but 

 a single genus and species, Opisthocomus cristatus, the 

 hoatzin of British Guiana, forms a well-marked group of 

 birds. 



The external characters of the adult and young have been 

 chiefly described in recent years by Nitzsch, Pyceaft,* and 



' In Corythaix persa the atlas ring is incomplete. 

 ' Cf. Oalli, in which the same fusion occurs. 



' B. Blanchakd, ' Bemarques sur rOst^ologie des Musophagides,' Compt. 

 Bend. xlv. p. 599. 



* ' On the Pterylography of the Hoatzin,' Ibis, 1895, p. 345. 



