1. Almost always possess an epidermic exoskeleton in the
form of scales or feathers.
2. The centra of the vertebrae are ossified, but have no
3. The skull has a completely ossified occipital segment,
and a large basisphenoid. No separate parasphenoid exists
in the adult. The prootic is always ossified, and either
remains distinct from the epiotic and opisthotic throughout
life, or unites with them only after they have anchylosed with
4. There is always a single, convex, occipital condyle, into
which the ossified ex-occipitals and basi-occiptal enter in various
5. The mandible is always present, and each ramus consists
of an articular ossification, as well as of several membrane
bones. The articular ossification is connected with the
skull by a quadrate bone. The apparent "ankle-joint" is situated,
not between the tibia and the astragalus, as in all Mammalia,
but between the proximal and the distal divisions of
6. The alimentary canal terminates in a cloaca.
7. The heart is trilocular or quadrilocular. Some of the
blood-corpuscles are always red, oval, and nucleated.
8. The aortic arches are usually two or more, but may be
reduced to one, which then belongs to the right side.
9. Respiration is never effected by means of branchiae, but,
after birth, is performed by lungs. The bronchi do not branch
dichotomously in the lungs.
10. A thoracic diaphragm may exist, but it never forms
a complete partition between the thoracic and the abdominal viscera.
11. The Wolffian bodies are replaced, functionally, by permanent
12. The cerebral hemispheres are never united by a corpus
13. The reproductive organs open into the cloaca, and the
oviduct is a Fallopian tube, whicb presents a uterine dilatation in the lower part ot its course.
14. All are oviparous, or ovoviviparous.
15. The embryo has an amnion, and a large respiratory
allantois, and is developed at the expense of the massive
vitellus of the egg.
16. There are no mammary glands.