286 STRUCTURE AND CLASSIFICATION OF BIRDS 



myself, 1 the internal anatomy of the soft parts by PERRIN, 2 

 GARBOD, a GADOW, 4 MITCHELL/' and myself. 



The pterylosis of Opisthocomus shows a less sharp differ- 

 entiation into pterylae and apteria than in many birds ; many 

 of the spaces are covered with a sparse feathering of semi- 

 plumes. This condition may be, as GARROD has suggested, 

 immediately derived from a continuous feathering. Never- 

 theless there is a sharply marked apterion upon the carina 

 sterni, which is necessarily, therefore, of limited extent. The 

 dorsal apterion is but feebly marked. In the young chick 

 the ventral apterion was as clear as in the adult, but I could 

 discover no trace of a dorsal bare space. In the young un- 

 hatched chick the ventral feathering was closer than the 



dorsal. 



The skin lying upon the carina sterni is dense and thick- 

 ened, a state of affairs which appears to have some relation 

 to its habit of squatting close to the branch upon which it is 

 resting. 



The principal recent papers upon the osteology of this 

 bird are those of HUXLEY G and the more recent and more 

 elaborate treatise of PARKER/ The cervical vertebra are 

 nineteen in the adult ; but PARKER found only eighteen in 

 the unhatched young. The atlas is notched for the odontoid 

 process. Hsemapophyses are very feeble. The last two or 

 three cervicals are ankylosed with each other, and form part 

 of the dorsal series of ankylosed vertebrae. There is some 



1 ' Contributions to the Anatomy of the Hoatzin,' &c., Ibis, 1889, p. 283. 

 See also C. G. YOUNG, ' On the Habits and Anatomy of Opisthocomus crixtatux ; ' 

 Notes Lcj/den Mus. x. p. 169 ; and .T. J. QUELCH, ' On the Habits of the 

 Hoatzin,' 'ibis, 1890, p. 327. 



2 ' On the Myology of Opisthocomus cristatus,' Tr. Zonl. Soc. ix. p. 353. 



3 ' Notes on Points in the Anatomy of the Hoatzin,' P. Z. S. 1879, p. 109. 



4 ' Description of the Modification of certain Organs,' &c., Zool. J.B. v. Abth. 

 Syst. v. 1891, p. 629, and Proc. R. Irish Ac. (3), ii. p. 147. 



r> See below, p. 288, footnote, for reference. See also a recent paper 

 by GOELDI in Ornith. M.B. May 1895, and more fully in Bol. Mus. Para. 



1895. 



B ' On the Classification and Distribution of the Alectoromorphss,' P. '/,. S. 



1868. 



On the Morphology of a Reptilian Bird, Opisthocomus cristatus,' Tr. '/.. S. 



1891. 



