Structurally related to the Notharctidae was the European Eocene family Adapidae. The skull was more robust than that of Notharctus, with higher crests, more massive cheek-arches and more expanded lower jaws. On the other hand, the lower molars were relatively smaller, with sharper oblique cross-crests. In many skeletal features the Adapidae approached the modern Mad agascan lemurs, to which they were either directly ancestral or closely related.
Madagascar has been the lemuroid headquarters for a long period. To-day it is the home of many species and genera of three families, while a number of recently extinct forms of widely differ ent genera have also been found there. The central family is the Lemuridae, including the true lemurs, which range in size from the tiny mouse-lemur (Microcebus) to the extinct short-limbed Megaladapis, as large as a half-grown brown bear.
The typical lemur has a fully arboreal skeleton, with grasping hands and feet, although certain species live in rocky places. The animal is essentially an arboreal quadruped, which runs and leaps on top of the branches. The head is fox-like, but the eyes are very large and protrude from their sockets. The ears are usually large and movable. The food consists of insects, fruits and leaves. The lower incisors and much reduced canines are sharply inclined forward, compressed and pointed. With these the animals comb their fur and seize their food. The upper molars have a rounded inner cusp connected by a sharp small cross-crest with the front one of the two rounded outer cusps. The opposite halves of the lower jaw are not fused in front as in the higher primates. The skull varies in length from the extremely long skull of Megalada pis to the short sloth-like skull of Myoxicebus. The placenta is diffuse, with a large allantois, and is indeciduous; i.e., is torn away from the uterine wall at birth.
The indrisine lemurs or sifakas, also peculiar to Madagascar, have more monkey-like faces. The central types have very long hind limbs and long hands and feet and are good climbers and leapers. The indris has a tail like that of a rabbit; the different species of Propithecus have brilliant areas of colour against a dark background. All are very conspicuous in a museum-case but as they are nocturnal animals their bizarre colouring may have a dis ruptive or concealing effect at night.
The skeleton could be derived directly from that of the Eocene lemuroids by emphasizing certain details almost to the point of caricature. These indrisine lemurs have a spiral colon, recalling that of other leaf-eating animals; the molar teeth also are adapted to cutting vegetation, the blades of each W-shaped lower molar fitting between corresponding V's of the upper molars. The area for the insertion of the masseter or outer jaw-muscle on the outer side of the lower jaw is greatly expanded and the bony cheek arches are strengthened to support the powerful grinding muscles. The lower front teeth are reduced to a single pair. The detailed construction of the skull is obviously a modification of the primi tive Eocene lemuroid type.
The extinct Archaeolemur (or Nesopithecus) resembled some of the South American monkeys in its upper and lower premolars and molars and in certain features of the skull, but in many other features (such as the middle ear region) it is connected with the indrisine lemurs. It seems probable that Archaeolemur illustrates one of the structural phases through which the ancestors of the New World monkeys may have passed in their descent from lemu roid ancestors. There is also a gradual transition in the form of the "braincast" and of the furrows on its surface between these lemuroids and the New World monkeys.
The last family of Madagascar lemuroids is that of the aye-aye (Daubentonia or Chiromys), one of the most extraordinary of all animals. Arboreal in habit, it has very large ears with which it listens for the sound of insect grubs boring beneath the bark. When it locates a grub it gnaws through the bark and into the wood with its rodent-like front teeth, which are large and com pressed with sharp tips. Into the narrow slot thus opened it thrusts its very long, attenuated third finger, hooking the grub on the end of its curved nail. In accordance with its grub- and fruit-. eating habits its molar teeth are much reduced. The general anat omy of the aye-aye agrees with that of other Madagascar lemu roids (especially the indris group). Fossil jaws with gnawing front teeth, much like those of the aye-aye, have been found in the French Eocene. These are more or less intermediate between the Eocene tree-shrews (Plesiadapidae) and the modern aye-aye. Probably the indrisine lemurs are a related off-shoot of the primi tive tree-shrew-lemur stock.
The lemurs of southeastern Asia and of Africa differ in certain respects from the Madagascar lemurs, but agree with them in the curious comb-like arrangement of the lower front teeth. The lorises (Lorisidae) extend from tropical India southeastward through most of Malaya and are also represented in west Africa. The slender loris (Loris gracilis) is a large-eyed, small-nosed, noc turnal form with long and excessively slender bent limbs and no tail. It clings to the branches and feeds upon leaves and fruit, small birds, insects and mice. The awantibo (Arctocebus) and the potto (Perodzcticus) of Africa have heavier skulls and blunter cusped molars. The hands are extremely specialized for grasping and have very large thumbs and vestigial second fingers.
The galagos of Africa (Galaginidae), commonly known as "bush babies," are active arboreal leapers, catching insects on the wing. They have long, pointed muzzles and very large eyes and ears. They approach the tarsioids in these characters and in the elongation of the tarsal bones for leaping. But they differ in the lemur-like character of the lower front teeth, and the resemblances between the two families seem partly due to parallelism. Pronyc ticebus from the French Lower Oligocene is represented by a fossil skull structurally intermediate between the Eocene adapids and the modern lorisids.