226 VERTEBRATA 



and in many species the hind ones were much larger than 

 the front ones, indicating that the animals walked on two 

 legs similar to the birds. The bones are hollow, as in birds, 

 and since in several other important features the skeleton 

 is similar to that of birds it is believed that the two groups 

 are closely related and have descended from a common 

 parent. Many species were of immense size. Bronto- 

 saurus, the giant reptile whose mounted stone skeleton is 

 in the Museum of Natural History in New York, was over 

 sixty feet long. Diplodocus, whose fossil remains may be 

 seen in the Carnegie Museum of Pittsburg, was about 

 seventy feet in length. The extinction of this entire group 

 of huge creatures may have been brought about by lack 

 of food or the presence of numerous parasitic or carnivor- 

 ous enemies. 



The reptiles are intermediate in structure between the 

 amphibians and birds. The brain resembles that of the 

 frog, but the cerebrum is larger. The vascular and respira- 

 tory systems approach the condition in birds. In most 

 forms the heart has two auricles and one ventricle* but 

 in the crocodile a partition divides the ventricle into two 

 parts. 



18. AVES 



The birds are distinguished from all other vertebrates 

 by the presence of feathers. The two anterior limbs are 

 modified into the form of wings adapted for flying or swim- 

 ming. They differ from the three preceding classes in 

 being warm-blooded, having a temperature of from one 

 hundred and two to one hundred and nine degrees. As an 



