278 REPTILIA CHAP. 



the Stegocephali, whilst on the other hand the Eeptiles, prob- 

 ably through some branch of the Theromorpha, have given rise 

 to the Mammals ; some other Eeptilian branch, at present 

 unknown, has blossomed out into the Birds. 



Principal Characters of the Reptilia. 



1. The vertebrae are gastrocentrous. 



2. The skull articulates with the atlas by one condyle, which is formed 



mainly Ijy the basioccipital. 



3. The mandilile consists of many pieces and articulates with the cranium 



through the quadrate bones. 



4. There is an auditory columellar apparatus fitting into the fenestra 



ovalis. 



5. The limbs are of the tetrapodous, pentadactyle type. 



6. There is an intracranial hjqtoglossal nerve. 



7. The ribs form a true sternum. 



8. The ilio-sacral connexion is post-acetabular. 



9. The skin is covered (a) with scales, but (b) neither with feathers nor 



with hairs ; and there is a great paucity of glands. 



10. Reptiles are poikilothermous. 



11. The red blood -corpuscles are nucleated, biconvex, and oval. 



12. The heart is divided into two atria and an imperfectly divided 



ventricle. It has no conus, but semilunar A-alves exist at the base 

 of the tripartite aortic trunk. 



13. The right and left aortic arch are complete and remain functional. 



14. Respiration is effected by lungs; and gills are entirely absent, even 



during embryonic life. 



15. Lateral sense-organs are absent. 



16. The kidneys have no nephrostomes. Each kidney has one separate 



ureter. 



17. There is always a typical cloaca. 



18. The eggs are meroblastic. 



19. Fertilisation is internal, and is effected, with the single exception of 



Sjjhenodon, by means of male copulatory organs. 



20. An amnion and an allantois are formed during development. 

 Numbers 1, 2, 6, 7, 8, 14, 16, 18, 20 separate the Reptiles from 



the Amphibia. Cf. also pp. 4 and 5. 

 Numbers 9 (b), 10, 12, and 13 separate them from the Birds and 



Mammals. 

 Numbers 3, 8, and 11 sej^arate them from the Mammals. 



The evolution of the classification of the Reptiles has 

 to a certain extent been already treated on pp. 7-9. For a 

 long time only Chelonia or Tortoises, Ophidia or Snakes, and 

 Saurii were recognised as their principal divisions. Then the 

 Crocodiles were separated from the Lizards; later the Coeciliae 

 were removed from the Snakes and referred to the Amphibia, 



