182 STRUCTURE AND CLASSIFICATION OF BIRDS 



We now come to the Tracheophonae, which are exclu- 

 sively American in range. Their distinguishing mark is, of 

 course, the tracheal syrinx, whose general structure has 

 been already explained (see p. 67). Besides these general 

 agreements the Tracheophonae show variations in structure. 

 Hetcrocnemis, for instance, is unique among them and 

 the Mesomyodi in general by the existence of a bilaminate 

 tarsus, as in the Oscines. Conopophaga and a few others ' 

 have a four-notched sternum, while Furnarius, Synal- 

 laxis, and a few others have, as has no other passerine, 

 a schizorhmal skull. Again, the maxillo-palatine of the 

 Dendrocolaptida3, Furnariidas, and Pteroptochidge are like 

 those of oscinine Passeres in being slender and curved back- 

 wards, instead of being comparatively wide and blunt, as in 

 other Mesomyodi. Pkytotoma is unique among passerine 

 birds for the nasal gland groove on the frontal bones, as in 'so 

 many water birds. 2 



The remaining group of Passeres, the Acromyodi, are 

 sometimes called Oscines and sometimes Polymyodi, the latter 

 term having reference to the numerous intrinsic muscles of 

 the syrinx. It was discovered by KEYSEELING and BLASIUS 

 that, with the exception of the Alaudidse, the Oscines have a 

 bilaminate tarsus, the hinder surface being covered by two 

 closely apposed scutes. It sometimes happens (in the forms 

 which are on that account spoken of as ' booted ') that the 

 anterior face of the tarsus is covered by a single scute. The 

 syrinx of these birds is complex in the multitude of its 

 muscles, of which there are four or five pairs. The only 

 exception to the muscle formula AXY exists in this group ; 

 in Dicrnrus there is the reduced formula of AX . Ocypterus, 

 the only passerine with powder down, is referable here. 

 Referable to this group, but separated from the more normal 

 members as ' Abnormales ' by GARROD, are the two genera 

 Atricliia and Menura, which are also regarded as the types 

 of separate families. These two anomalous birds are by 



1 FORBES, ' On some Points in the Anatomy of the Genus Conopophaga,'' &c., 

 P. Z. S. 1881, p. 485. 



- PARKEK, ^githognatlious Birds, ii. p. 258. 



