On The Origin of Species by Charles Darwin Chapter 16 Page 26

(MAMMAE, MAMMARY GLANDS) of the mother. A striking difference in embryonic development has led to the division of this class into two great groups; in one of these, when the embryo has attained a certain stage, a vascular connection, called the PLACENTA, is formed between the embryo and the mother; in the other this is wanting, and the young are produced in a very incomplete state.

The former, including the greater part of the class, are called PLACENTAL MAMMALS; the latter, or APLACENTAL MAMMALS, include the Marsupials and Monotremes (ORNITHORHYNCHUS).

MAMMIFEROUS. — Having mammae or teats (see MAMMALIA).

MANDIBLES. — in insects, the first or uppermost pair of jaws, which are generally solid, horny, biting organs.