If our diagnosis is to include the fossil Reptiles we have not only to discard the characters drawn from the soft parts as unavailable, but we are forced to treat the condition of the occipital condyle with caution, since there exist, or must have existed, transitional stages between Reptiles and Amphibia and Mammals; and the winged class Pterosauria does not permit us to use the wings as a differential character for the Birds. In fact, while the Reptilia are sufficiently separated from the Amphibia by their absolutely gastrocentrous vertebrae, it is difficult to distinguish them as a class from the Birds; hence the term SAUROPSIDA, which is intended to indicate the close relationship of the Reptiles to the Birds in opposition to the Mammalia, and to the ICHTHYOPSIDA or Amphibia and Fishes. However, the Reptilia take up a very central position in the evolution of the main classes of the Vertebrata. On the one hand, there is not the slightest doubt that they are evolved from some branch of {278}the Stegocephali, whilst on the other hand the Reptiles, probably through some branch of the Theromorpha, have given rise to the Mammals; some other Reptilian 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 by the basioccipital. 3. The mandible 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 hypoglossal 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 valves 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 _Sphenodon_, 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.