Fossil Lemurs.—The Lemuroids are a very ancient race; they extend back to the very earliest strata of the Eocene, the Torrejon and Puerco beds, which, as already said, are thought to be more referable to the Cretaceous than to the Tertiary epoch.
One of these early forms is referred to the genus Mixodectes, a genus which has been placed, though with a query, in the order Rodentia. It appears, however, to be a Lemuroid, and is of American range. The incisor teeth have been held to argue that it lies on the direct track of Chiromys; but other features, more especially the form of the astragalus, have been used to argue the justice of the inclusion of this type within the order Rodentia. Allied, as it is supposed, to this form is Indrodon, also of the lowest Eocene deposits of the United States. Indrodon malaris is known from fragments of nearly all parts of the skeleton. They indicate the existence of a creature of about one-half the size of Lemur varius. It had slender limbs and a long and powerful tail. The humerus, as in so many archaic beasts, has an entepicondylar foramen. The femur has three trochanters, and the fibula articulates with the astragalus. It is not always easy to distinguish these primitive mammals from each other, so that the minutest of characters have to be called in to our assistance. One of the contemporaneous groups with which these early Lemurs might be confused is that of the Condylarthra; it is important, therefore, to note that in Indrodon the calcaneo-cuboidal articulation is nearly flat, and not bent as it is in the former group. The teeth are of the tritubercular pattern. The incisors are not known, but the molars and premolars are each three. To the same family, which has been termed Anaptomorphidae, is referred the genus Anaptomorphus, which has been specially compared to Tarsius. This small animal has a Lemurine face with huge orbits. It has a premolar less than Indrodon. It has been ascertained that A. homunculus had an external lachrymal foramen.[[412]]
Another family, that of the Chriacidae, appear to hover on the border line of Lemurs and Creodonts, having been referred to both by various palaeontologists. Professor Scott suggests their Lemurine or at least Primate relationships, while Cope urged their Creodont affinities. A difficulty raised by Scott was, that in Chriacus the premolars of the lower jaw were spaced. But it appears that this is not fatal to their inclusion in the Primates, since Tomitherium, an "undoubted Primate," shows the same feature. If Chriacus is a Lemur it is an earlier type than those
which have been considered; for it has the typical Eutherian dentition of four premolars and three molars. These teeth, especially the superior molars, are particularly compared to the corresponding teeth of Lemur and Galago. Of this and the allied genus, Protochriacus, several species are known.
Adapis, a representative of another family, is one of the best known of ancient Lemuroids. It has the typical mammalian dentition of forty-four teeth in a close series without diastemata. The orbits are completely separated from the temporal cavity, the eyes looking forwards. The canines are large and caniniform. The skull is deeply ridged behind with the usual sagittal crest. This genus is European, and corresponds to the already mentioned American Eocene Tomitherium, perhaps belonging to the same family.
Nesopithecus is an extinct genus from Madagascar, lately described by Dr. Forsyth Major.[[413]] There are two species, N. roberti and N. australis. The dental formula is I 2, C 1, Pm 3, M 3, for the upper jaw, the lower jaw having but a single pair of incisors. The lachrymal foramen is just inside, or on the edge, of the orbit, so that one distinctive Lemurine character is lost. The genus is also Ape-like in the form of the canines and incisors, these having been especially compared by Dr. Forsyth Major with those of the Cercopithecidae. The molars, too, agree with those of the same family. There is, however, one important feature in which Nesopithecus resembles not only the Lemurs as opposed to the Apes, but the Malagasy Lemurs. As already mentioned (p. [544]), Dr. Major has shown that in the Malagasy Lemurs, even including the aberrant Chiromys, and in the Tertiary and European Adapis, the bulla tympani is not produced by an ossified extension of the annulus tympanicus, but from the adjacent periotic bone, the annulus remaining separate and lying within the fully-formed bulla. This feature shows conclusively that Adapis is a Lemur, and that Nesopithecus, originally supposed to be a Monkey, cannot be removed from the Lemuroidea, many though its likenesses to the higher Primates undoubtedly are. However, this feature, combined with the fact that the orbital and temporal cavities are in communication, shows the Lemuroid position of Nesopithecus, though it is quite conceivable that it is on the way to become an Ape.
A family, Megaladapididae, has been quite lately founded by Dr. Forsyth Major[[414]] to include the remains of a gigantic extinct Lemur from Madagascar, which when alive, so far as we can judge from the skull, must have been three or four times the size of the Common Cat. The name Megaladapis madagascariensis was given to the fossil on account of certain resemblances to the also extinct Adapis. It differs from other Lemurs in a number of characters which jointly warrant its inclusion in a distinct family. The small size of the orbits suggest a diurnal life; the deep mandibles, which, unlike what is found in other Lemurs, are completely blended at the suture, point to the existence of a howling apparatus, as in Mycetes. The low brain-case is a character which is found in so many extinct Mammalia belonging to many different orders that it weighs neither one way nor the other in considering the systematic position of the animal. The shape of the molars, which are three in each half of each jaw as in other Lemurs, is, according to the discoverer, like that of the genus Lepilemur. The incisors and the canines are not known. Of a still larger form, M. insignis, the molar teeth are known.[[415]]
Sub-Order 2. ANTHROPOIDEA.
The Apes differ from the Lemurs in that the teats are always restricted to the thoracic region; the orbit, though surrounded by bone as in the Lemurs (and in Tupaia, a very Lemur-like Insectivore), does not open freely behind into the temporal fossa as in Lemurs (except Tarsius). The lachrymal opening is inside the orbit instead of outside; the cerebral hemispheres are more highly developed, and conceal, or nearly conceal, the cerebellum; the upper incisors are in close contact; a few other points are mentioned under the description of the characters of the Lemurs. There are altogether about 212 species of Monkeys and Apes. They are tropical and subtropical in range, and, with but few exceptions, are impatient of cold.