232 BACKBONED ANIMALS. 



that passes so much of its time under water, has a similar 

 arrangement by which it is enabled to see equally well in 

 either element. There is, as a rule, no external ear. The 

 nostrils are slits situated in the upper portion of the man- 

 dibles, and are protected with bristles or scales. The 

 songs of birds constitute their language, this being par- 

 ticularly evident in the common fowl. The sounds made 

 when a hawk approaches, when an egg has been laid, 

 when calling their young, and the " song " on warm days 

 when in search of food, are all different expressions of 

 totally different emotions. The songs of birds have 

 been set to music by Mr. X. Clarke (see " American Natu- 

 ralist," vol. xiii, page 12). 



Development. All birds are oviparous. The eggs are 

 either hatched by the male or female, or both, or by the 

 sun (gulls), or artificial heat (brush-turkey). The young 

 are generally provided with a calcareous knob upon their 

 soft bill, as in some reptiles, with which they break the 

 egg and escape. Some are at first helpless, and have to 

 be fed, while others run (chickens) or fly (maleo) imme- 

 diately upon their escape. About seven hundred species 

 of birds are found in North America north of Mexico, and 

 in all about eight thousand distinct species are known 

 throughout the world, of which the following are some of 

 the most typical and interesting examples : 



Sub-Class I. LIZARD-TAILED BIRDS (Saurttrce). 



The first and lowest forms of birds were extremely 

 reptilian in their characteristics. The Archozopterix is a 

 remarkable fossil form found in the Jurassic slates at 

 Sohenhofen, Germany. It attained the size of a crow ; 

 the beaks were armed with conical teeth ; the tail was 

 formed by a long extension of the vertebrae, the feathers 

 growing out upon the sides, and the wings were bird-like 

 They are all extinct. 



