Of the _pre-molars_ the anterior are canine-shaped, the median and
posterior ones have three main, and one or two subordinate, cusps on the
crowns. In both the upper and lower _molars_, cross-bridges stretch between
the outer and inner front cusps as well as between the outer and inner hind
cusps. {11}There is an oblique ridge between the hind outer and the front
inner cusp, and another is often present between the front outer cusp and
the anterior "heel," producing, as Huxley has pointed out, almost a double
crescentic pattern, as in many lower Mammals. The posterior molar has four
or five cusps.
Of the _milk-teeth_, the incisors in the upper jaw change first. Of the
molars, two are developed before the change of the pre-molars. In the lower
jaw the incisors change first, and when two or three pre-molars have
developed the last molar has still to come.
The arm-bone, or _humerus_, has one perforation (_entepicondylar foramen_)
on its inner margin, and another above the joint (except in
_Perodicticus_). The bones of the fore-arm (_radius_ and _ulna_), and those
of the leg (_tibia_ and _fibula_) are not co-ossified (except in
_Tarsius_), so that the palm or sole can be turned up at will.
The bones of the _digits_ are more or less flat and rounded at the tips
(differing in this respect from the _Insectivora_). One of the ankle-bones,
for the articulation of the opposable great toe, the ento-cuneiform, as it
is called, is rounded, as in the Anthropoid Apes and Man. The thumb is
opposable, but its articulating bone in the wrist is not rounded, except in
_Avahis_ and _Indris_, which genera agree in this respect with
_Anthropopithecus_ and Man. The wrist has its central bone (_os centrale_)
present; it is absent in Man and the higher Apes.
The knee is free and not united to the side of the body by integument.
The two halves of the lower jaw are not always co-ossified (as is the case
in the _Anthropoidea_).
The opening in the base of the skull (the _foramen rotundum_) which
transmits from the brain a branch of the fifth nerve {12}for the upper jaw,
and the sphenoidal fissure, which gives exit to the third, fourth and sixth
cranial nerves, have but one aperture, as in the Rabbit, which belongs to
the _Rodentia_.
The sacral vertebræ are generally three in number, and the lumbar and
dorsal together vary from nineteen to twenty-three.
The brain, as Sir William Flower has observed, departs considerably from
the form of what may be called the primatial type, and approaches in form
to that of the carnivorous animals. The hind-brain, or _cerebellum_, is not
completely covered by the cerebrum. The latter has but few convolutions
(indicating a low intelligence), but its posterior lobe is always present,
though more or less rudimentary, and so also are many fissures, which are
characteristic of its surface in the higher Primates. The olfactory lobes
are usually large and not covered by the cerebrum.
The uterus and structures for the nutrition of the young prior to birth are
low in type, and approximate to the conditions seen in the Pig, the Horse,
the Chevrotains, and the Ruminants. The unborn Lemur is often encased (as
among the Sloths) in a skin-like covering (_epitrichium_) which breaks into
patches before birth.
The tongue has a horny supplementary under-tongue (_sublingua_) attached
beneath it. The stomach is simple, not formed of several compartments. The
transverse portion of the great intestine is convoluted in a remarkable
manner upon itself, the cæcum also being very large. The main arteries of
the arm and leg break up (as in the Sloths) into an immense number of small
vessels (called _retia mirabilia_) parallel to one another instead of being
simple branching trunks.
The long tendons of the muscles for flexing the digits (the {13}_flexor
longus digitorum_) differ generally in arrangement from those of the higher
Primates.
The Lemuroids are of no commercial value to Man.
As regards their distribution, the _Lemuroidea_ are now absolutely confined
to the Old World, and predominate in the island of Madagascar, where, as M.
Grandidier remarks in his magnificent work on that country, there is
scarcely a little wood in any district in which they are not found. Indeed,
of the nearly seventy species of Mammals inhabiting that island,
thirty-five, or one-half, are Lemurs. Members of the family also occur
across the whole of the neighbouring continent of Africa, but their
northern range does not reach quite to the tropic, whereas it extends some
few degrees beyond it in the Southern Hemisphere. Elsewhere they are
confined to the forests of the Oriental region. More or less isolated in
Southern India, they re-appear in China, and spreading south to Java they
reach as far east as Celebes and the Philippine Islands. The present
isolation of the Lemurs in two such distant areas--in Africa and Madagascar
and some of the Mascarene Islands on the one hand, and in Southern India,
China, Ceylon, and the Malayan Islands on the other--has been considered by
some naturalists as weighty evidence in favour of a former land connection
between these distant regions.
Though so restricted in their distribution at the present day, this group
was more widely represented in past ages of the world's history, as we
shall have to point out later on. Abundant fossil remains prove that they
lived in Europe and in North America, where to-day they are quite unknown.
The _Lemuroidea_ are almost entirely arboreal, and seldom come to the
ground, except the Sifakas, which then progress {14}on their hind legs by a
series of bounds, holding their hands over their head in a ludicrous
fashion. Most of them are nocturnal, or crepuscular, sleeping the greater
part of the day in holes or on a branch of a tree coiled up in a ball.
Their food consists chiefly of leaves, fruits, honey, birds' eggs, and
birds, or any small animals they can pounce upon.
The Lemurs now living are divided into three families. The Aye-Aye and the
Tarsiers, on account of their very special characters, constitute each a
distinct family--named _Chiromyidæ_ and _Tarsiidæ_ respectively--while the
True Lemurs form the third, the _Lemuridæ_, to which all the remaining
forms belong.