VERTEBRATA. 3138 
are numerous gigantic Saurians, which can not be classi- 
fied with any of the four living orders. Such are the 
Ichthyosaurus, Plesiosaurus, Pterodactyle, Megalosau- 
rus, and L[guanodon. 
Crass [V.—<Aves. 
Birds form the most clearly defined class in the whole 
Animal Kingdom. The Eagle and Hummer, the Ostrich 
and Duck, widely as they seem to be separated by size, 
form, and habits, still exhibit one common type of struct- 
ure. On the whole, Birds are more closely allied to Rep- 
tiles than to Mammals. In number, they approach the 
Fishes, ornithologists having determined 11,000 species. 
A Bird is an air-breathing, egg-laying, warm-blooded, 
feathered Vertebrate, with two limbs (legs) for perching, 
walking, or swimming, and two limbs (wings) for flying 
or swimming. Organized for flight, it is gifted with a 
light skeleton, very contractile muscular fibre, and a re- 
spiratory function of the highest development. 
The skeleton is more compact than those of Reptiles 
and Mammals, at the same time that it is lighter, and the 
bones are harder and whiter. It contains fewer bones 
than usual, many parts being anchylosed together, as the 
cranial, dorsal, and sacral. The lumbar vertebrae are 
wanting; but the neck is remarkably long (containing 
from 9 to 24 vertebra) and flexible, enabling the head to 
be a most perfect prehensile organ. The ribs generally 
are jointed in the middle, as well as with the backbone 
and sternum. The last, where the muscles of flight orig- 
inate, is highly developed.'* The skull articulates with 
the spinal column by a single condyle, and with the low- 
er jaw, not directly, as in Mammals, but through the in- 
tervention of a separate bone, as in Reptiles. 
All Birds always have four limbs, while every other 
vertebrate class shows exceptions. The fore-limbs are fit- 
