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394 DE2sTAL ANATOMY.

It will be seen, from the foregoing table, that the IMammalia
are divisible into two primary groups, which hold the rank of sub-
classes. The first of these, Protothcria,^ has but tv,o living repre-
sentatives, both of which are confined to the continent of Australia.
These are the Echidna, or spiny ant-eater, and the duck-billed j)latypus.
The principal characters by which they are separated from all other
Mammalia may be conveniently contrasted with those of the second
sub-class, Eatlieria, as follows : in the former there are (1) '' large and
distinct coracoid bones, which articulate with the sternum. (2) The
ureters and the genital ducts open into a cloaca into which the urinary
bladder has a separate opening. (3) The penis is traversed by a ure-
thral canal which opens into the cloaca posteriorly, and is not continuous
with the cystic urethra. (4) There is no vagina. (5) The mammary
glands have no teats." In the Eutlier'm, on the other hand, (1) " the
coracoid bones are mere processes on the scapula in the adult, and do
not articulate with the sternum. (2) The ureters open into the bladder,
the genital ducts into a urethra or vagina. (3) The cystic urethra is
continuous with the urethral canal of the penis. (4) There is a single
or double vagina, (o) The mammary glands have teats " (Huxley).
In their anatomical structure the I'rotothoia resemble the rc})tiles and
birds more than does any other mannnal. This is particularly conspic-
uous in the pectoral arch and the repi'ocluctive system. On this account,
De Blainville applied the name OrnithocMjjliki (bird womb) to them,
by which they are sometimes known. Strange as it may seem, no fossil
remains of great anti(piity of this most primitive grouj) of all Mammalia
are with certainty known to exist, but it may. yet be found that the
earliest mammalian representatives, which date as far back as the
Triassic Period, and which are known from teeth and jaw bones
only, really belong to the Protofhcria rather than to the Didclpliia or
pouched series of the Eutheria, as is frequently maintained. Both
the living forms are devoid of true teeth.
The second sub-class, Eutheria, has two principal divisions : Didelphia
(double womb), including those animals popularly known as the
" pouched cpiadrupeds," of Avhich the opossum, kangaroo, wombat,
etc. are familiar .examples; and the MonodelpJiia (single womb),
which embraces all the remaining mammals. The name of the first
subdivision, Didelphia, was applied by its author, De Blainville, with
reference to the peculiar habit which these animals possess of sheltering
their helpless young in an abdominal integumentary fold. This is corre-
lated with the only important character in which they differ from the
monodelph division—viz. the complete absence of an allantoic placenta
or any uterine connection between mother and foetus. In consequence
of this peculiarity of gestation the young are born in an exceedingly
helpless and imperfect condition, and are nourished for a considerable
period in the marsupium or pouch of the mother. This character is

^ The classification of tlie jNTammalia proposed recently by Prof. Hnxley includes
three jjrincipal subdivisions— viz. : Prololheria, JSIelailieria, and Fjilheria. The terms
Pfotnlheria and Eutlirria were employed by Prof. Gill a number of years previously t<>
desisnate the two ])rincipal groups of this class, and appear to have been appropriated
bv Huxlev without credit.