VERTEBRATA AMPHIBIA. 45 



47. Nervous System. The brain is relatively smaller 

 than in the birds or mammals. The surfaces of the hemi- 

 spheres of the cerebrum are without convolutions. There 

 is no corpus callosum. 



48. Covering. The covering of reptiles consists of horny 

 scales developed by the epidermis (Gr. epi, upon ; derma, 

 the skin) or cuticle ; and in the crocodiles, tortoises, and 

 turtles, there are also bony plates, formed by the derma 

 or true skin. 



49. Development. All reptiles are oviparous. The 

 outer coating of the egg is usually leathery, instead of 

 being calcareous as in the birds. Sometimes the egg 

 undergoes the necessary incubation within the body 

 of the mother, so that the young is developed before 

 the exclusion of the egg. Such reptiles are said to be 

 ovoviviparous, 



CLASS IY.- Amphibia. 



50. The term Amphibia (Gr. amphi, both; bios, life) was 

 formerly applied to aquatic mammals, such as the seals ; 

 but it is now restricted to a group of animals, including the 

 frogs, toads, and newts, which at one period of life are 

 adapted for breathing in water, and afterwards for breath- 

 ing in air. 



The Amphibia and the fishes form the section of the 

 Yertebrata termed Ichthyopsida. They resemble fish in 

 the following details: They are oviparous; the embryo 

 is destitute of the membranes termed amnion and 

 allantois; gills or organs adapted for breathing the air 

 dissolved in water are always present at some period 

 of life; there is no diaphragm; the red blood-cor- 

 puscles are oval and nucleated; there are no mammary 

 glands. 



51. Skeleton. The skull, as in the mammalia, unites 

 with the vertebral column by two occipital condyles; but 

 the " basi-occipital" is cartilaginous not bony, as in the 

 higher Yertebrata. There are two corresponding cups in 



