The Mammalia

1. Always possess an epidermic exoskeleton in the form of hairs.
2. The vertebrae are ossified, and (except in the ornithodelphia) their centra have terminal epiphyses.
3. All the segments of the brain-case are completely ossified. No distinct parasphenoid exists in the adult. The prootic ossifies, and unites with the epiotic and opisthotic before these coalesce with any other bone.
4. There are always two occipital condyles, and the basioccipital is well ossified.
5. The mandible is always present, and each ramus consists (at any rate, in the adult) of a single membrane bone, which articulates with the squamosal. The quadrate bone, and the supra-stapedial element of the hyoidean arch, are converted into a malleus and an incus, so that, with the stapes, there are, at fewest, three ossicula auditus.
6. The alimentary canal may, or may not, terminate in a cloaca. When it does not, the rectum opens behind the genito-urinary organs.
7. The heart is quadrilocular. Some of the blood-corpuscles are always red and non-nucleated.
8. There is only one aortic arch which lies on the left side.
9. Respiration is never effected by means of branchiae, but, after birth, is performed by lungs.
10. There is a complete diaphragm.
11. The Wolffian bodies are replaced by permanent kidneys.
12. The cerebral hemispheres are united by a corpus callosum.
13. The reproductive organs may, or may not, open into a cloaca. The oviduct is a Fallopian tube.
14. The embryo has an amnion and allantois.
15. Mammary glands supply the young with nourishment.