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.—Lizarp-TaILeD Brrps (Saurure). 
The first and lowest forms of birds were extremely 
reptilian in their characteristics. The Archaopterix 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 vertebra, the feathers 
growing out upon the sides, and the wings were bird-like. 
They are all extinct. 
