3. There is an auditory columellar apparatus, fitting into the fenestra ovalis. 4. The limbs are of the tetrapodous, pentadactyle type. 5. The red blood-corpuscles are nucleated, biconvex, and oval. 6. The heart is (_a_) divided into two atria and one ventricle, and (_b_) it has a conus provided with valves. 7. The aortic arches are strictly symmetrical. 8. Gills are present at least during some early stages of development. 9. The kidneys are provided with persistent nephrostomes. 10. Lateral sense-organs are present at least during the larval stage. 11. The vagus is the last cranial nerve. 12. The median fins, where present, are not supported by spinal skeletal rays. 13. Sternal ribs and a costal or true sternum are absent. 14. There is no paired or unpaired medio-ventral, copulatory apparatus. 15. Development takes place without amnion and allantois. None of these characters is absolutely diagnostic, except 1 (_c_), and this applies only to the Anura and most of the Stegocephali. {5}Numbers 1 (_b_), 1 (_c_), 2, 3, 4 and 12 separate the Amphibia from the Fishes. Numbers 1, 6 (_b_), 7, 8, 9, 11, 13, 15 separate them from the Reptiles, Birds, and Mammals. Number 2 separates them from the Fishes, Reptiles, and Birds. Number 5 separates them from the Mammals. Number 6 (_a_) separates them from the Fishes (excl. Dipnoi), Birds and Mammals. We can, therefore, very easily define all the Amphibia, both recent and extinct, by a combination of the characters enumerated above. For instance, by the combination of numbers 2, 3 or 4 with either 7, 8, 9, 11, 13 or 15. _Amphicondylous Anamnia_ would be an absolutely correct and all-sufficient diagnosis, but it would be of little use in the determination of adult specimens; and the tetrapodous character is of no avail for Apoda. _Amphicondylous animals without an intracranial hypoglossal nerve_ is a more practical diagnosis. In the case of living Urodela and Anura the absence of any scales in the skin affords a more popular character; it is unfortunately not applicable to the Apoda, many of which possess dermal scales, although these are hidden in the imbricating transverse rings of the epidermis; and the frequent occurrence of typical scales of both ecto- and meso-dermal composition in many of the Stegocephali forces us to discard the scales, or rather their absence, as a diagnostic character of the class Amphibia. The same applies to the mostly soft, moist, or clammy, and very glandular nature of the skin.