38 THE VERTEBRATA OR VERTEBRATES. 



For our present purposes, we may divide the Vertebrates 

 into five Classes, as follow : 



I. MAMMALIA, or air-breathing, warm-blooded animals, which 

 bring forth living young, and nourish them with milk ; which 

 usually have the ekin covered with hair, fur, or wool ; lungs 

 suspended in a cavity separated by a diaphragm from the 

 cavity of the abdomen, and unconnected with air-saes ; a heart 

 'with four cavities; a complete and double circulation of the 

 blood; blood-corpuscles non-nucleated, and in nearly all 

 cases circular ; the skull with two occipital condyles ; the 

 lower jaw composed of only two pieces, and articulated di-: 

 rectly with the skull without the intervention of a quadrate 

 bone ; and teeth generally present, enameled, and certain 

 kinds implanted by two or more roots ; as Man, Beasts of prey, 

 our domestic quadrupeds, etc. 



II. AVES, or Birds, or air-breathing, warm-blooded animals, which 

 lay eggs from which by brooding they hatch their young ; 

 which have their skin covered with feathers ; lungs con- 

 nected with air-sacs, and not separated from the cavity of the 

 abdomen by a diaphragm ; a heart with four cavities ; a 

 complete and double circulation of the blood ; blood-cor- 

 puscles, oval and nucleated ; the skull with only one occipital 

 *condyle; each half of the lower jaw composed of several 

 pieces and articulated to the skull by means of a quadrate 

 bone ; and which in all cases are destitute of true teeth. 



III. REPTILIA, or air-breathing, cool-blooded animals, which lay 

 eggs from which their young are hatched without incubation ; 

 which have the body covered with plates or scales ; the lungs 

 not connected with air-sacs, and not separated from the ab- 

 dominal cavity by a diaphragm ; a heart with three or four 

 cavities ; the blood-corpuscles oval and nucleated ; the skull 

 with one occipital condyle ; each branch of the lower jaw 

 composed of several pieces, and articulated to the skull by 

 means of a quadrate bone ; and teeth with only one root, or 

 set in a groove, or wanting ; as Turtles, Saurians, and Snakes. 



IV. BATRACHIA, or Amphibians, or cool-blooded animals, which 

 breathe by gills in the young state, but which afterwards have 

 lungs (in some kinds both lungs and gills), and thus be- 

 come air-breathers ; which lay their egg in the water, or iu 



