278 AVES ciAss IV 



Order 13. PASSERES. 



The Order Passeres embraces about one-half, or, in round numbers, 6000 

 species of existing birds, mostly of small size, the raven being the giant of the 

 Order, widely distributed over the globe. The skull is aegithognathous ; ^ the 

 sternum bears a Y-shaped manubrium, and is deeply two-notched behind 

 (Compophaga and a few others are four-notched). The hypocleidium (inter- 

 clavicle) is well developed save in Menura and Atrichia, and almost rests upon 

 the anterior part of the sternal keel. 



With^the Single exception of Cholornis, in which the fourth toe is a m^e 

 vestige, there are three toes in front and one behind ; the hypotarsus is more 

 highly specialised than in any other group, having four large tendinal 

 perforations, and sometimes two smaller ones in addition ; the typical number 

 of presacral vertebrae is nineteen, fourteen of which are cervical, the 

 Australian genus Petroica being exceptional in having thirteen. The skeletal 

 variations in all this great group of birds are small, so small indeed that it is 

 almost impossible to use them in making subdivisions, external differences, 

 such as slight variations in the proportions of the toes and form of the beak 

 being used for this purpose. , 



It is instructive to note that Huxley and Fürbringer, who used skeletal 

 characters, made two families of the Passeres, while Sharpe, using external 

 characters, makes forty-nine. The Australian lyre birds (Menuridae), however, 

 form a distinct group, and so do the Asiatic broadbills {Eurylaimidae), as 

 well as the American tyrant flycatchers (Tyrannidae), and their relatives ; 

 also the ant thrushes (Formicariidae), and their relatives. The order dates 

 from the Eocene, Palaeospiza^ coming from the Florissant Shales of Wyoming, 

 and Palaegithalus and a starling (LauriUardia) from the Eocene of the Paris 

 Basin. Above that horizon more occur, but the small size of most Passeres 

 is a good reason for the lack of discovery, or lack of preservation, of many 

 species. 



In conclusion, it may be well to repeat that our knowledge of the phylogeny 

 of the various groups of birds is very imperfect, and that most of the fossil 

 forms are not only representatives of well-known families, but often belong to 

 the same genera as species now living in the same geographic area in which 

 the fossils occur. This is in marked contrast with what we find in the 

 Mammalia, for not a family of these found in the Eocene, and few from the 

 Miocene, are represented by living species. 



From the fact that the majority of birds possess the power of flight they 

 throw little light on the problems of former land connections and lines of 

 distribution, although they may to some extent indicate climatic differences 

 between the past and the present. There is a great gap between the birds of 

 the Eocene and the toothed birds of the Cretaceous, and a greater one between 

 these and the Jurassic Archaeopteryx, while the point at which birds diverged 

 from reptiles, and put on their dress of feathers, is wholly unknown. 



[The preceding chapter on Avcs has been revised, and in large part rewritten by Mr. 

 Frederic A. Lucas, of the United States National Museum at Washington, D.O. — Editor.] 



1 Parker, W. K., On the Skull of Aegithognathous Birds (Trans. Zool. Soc. London, Part L vol. 

 IX. pp. 289-352 ; Part IL vol. X. pp. 251-314), 1873, 1878. 



- This specimen seems to have been lost, but one is inclined to doubt from the figure that this 

 bird is truly a passerine. 



