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  Section: Anatomy of Vertebrate Animals » The Classification and Organization of the Mammalia
 
 
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The Lemuridae

 
     
 

a. The first of these divisions, the Lemuridae, is more widely separated, anatomically, from the other two, than these are from one another, (On the strength of these differences M. Gratiolet relegated the Lemurs to the Insectivora; and Mr. Mivart, in his valuable paper "On the Axial Skeleton in the Primates," publislied in the Proceedings of the Zoological Society for 1855, divides the Primates into two sub-orders, Lemuroidea and Anthropoidea.) and it contains some forms which very closely approximate to the Insectivora, while others are nearly affined to the Rodentia.

All the Lemuridae are habitually quadripedal, have the integument furry, and are usually provided with long tails which are never prehensile. They are devoid of cheekpouches and of callous patches upon the integument covering the ischia.

The fore-limbs are shorter than the hind-limbs. In the foot, the hallux is large and opposable, and the second digit differs from the rest in size, and in the claw-like form of its nail. The fourth digit is usually longer than the others, the difference being especially marked in the pes.

In the skull, the brain-case is small relatively to the face, and is contracted anteriorly. If a straight line drawn from a point midway between the occipital condyles, through the median plane of the skull, to the junction of the ethmoid and presphenoid, in the floor of the cerebral cavity, be termed the basi-cranial axis; and if the planes of the cribriform plate of the ethmoid, of the tentorium cerebelli, and of the occipital foramen, be respectively termed the ethmoidal, tentorial, and occipital planes; then, the greatest length of the cerebral cavity hardly exceeds the length of the basi-cranial axis; and the ethmoidal, tentorial, and occipital planes are very much inclined to that axis. The upper aperture of the lachrymal foramen lies upon the face, outside the front margin of the orbit. The frontal and the jugal bones are united behind the orbit, but a mere bar of bone results from their union; and it is so narrow that the orbit and the temporal fossa are in free communication. The bony palate is elongated, and, in many species, its posterior free edge is thickened.

The lateral processes of the atlas are, usually, expanded. The lumbar region of the spine is elongated; the vertebrae composing it, in some cases, being as many as nine. There are nine bones in the carpus. The ilia are narrow and elongated, and the ischia are not everted. In most Lemurs, the tarsal bones resemble those of the other Primates; but, in Otolicnus and Tarsius, they have undergone a modification, a parallel towhich is not to be found among Mammals, but must be sought among the Batrachia. When the distance between the heel and the digits is great in other Mammalia, the elongation affects the matatarsal bones and not the tarsus; but, in these Lemurs, the calcaneum and the naviculare are prolonged, as they are in the Frogs.

The sublingua, a process of the mucous membrane of the floor of the mouth, developed between the apex of the tongue and the symphysis of the mandible, acquires a considerable size, and is often denticulated, or comblike, at its free end. The stomach is simple, with the cardiac and pyloric apertures approximated. The caecum is long, and has no vermiform appendage.

In many Lemurs (Stenops, Nycticebus, Perodicticus, Arctocebus, Tarsius) the great arteries and veins of the limbs break up into retia mirabilia formed of parallel branches.

The ventricles of the larynx may be enlarged, but there are no great air-sacs, such as exist in many other Primates.

In the brain, the cerebral hemispheres are relatively small and flattened, and have narrow and pointed frontal lobes. They are so short as to leave the cerebellum largely uncovered. The gyri and sulci are scanty, or absent, upon the outer surface of the hemispheres, but the internal face exhibits the calcarine sulcus. The large olfactory lobes project forward beyond the cerebral hemispheres.

The pendent penis of the male commonly contains a bone; the testes are lodged in a more or less complete scrotum; and vesiculae seminales are generally present.

In the female, the uterus has two long cornua, and the urethra traverses the clitoris. Sometimes there are one or two pairs of teats on the abdomen, in addition to the ordinary pectoral pair.

The Lemuridae are distinguishable into two families, the Lemurini and the Cheiromyini.

In the Lemurini, the pollex is large, opposable, and almost always has a broad, flat nail.

The usual dental formula is i. 2.2/2.2 c. /1.1 p.m. m. 5.5/5.5, or 6.6/6.6.

The upper incisors are vertical, and the pairs of opposite sides are generally separated by an interval. The upper canines are large and pointed, and very different from the incisors. The lower incisors are close set, laterally compressed, long and proclivous, and the canines, which resemble them in form and direction, are closely applied to the outer incisors. When six grinders are present, the anterior three are premolars. The anterior premolars, and sometimes all of them, have triangular and sharp-pointed crowns; the first premolar of the lower jaw, in fact, resembles a canine, but its true nature is shown bv its biting behind the upper canine, not in front of it.

Very generally the crowns of the upper molars are quadricuspidate, and an oblique ridge passes from the antero-external to the postero-internal cusp, as in the highest Primates; while, in the lower jaw, there are either two transverse ridges, or longitudinal crescents. The cusps of the molars are usually much produced, as in the Insectivora.

In the Cheiromyini, the pollex is not truly opposable, and its nail is claw-like and resembles that of the other digits. All the digits of the pes, except the hallux, have compressed, claw-like nails. The middle digit of the manus is much more slender than any of the others, and is longer than the fourth. The long axis of the articular head of the mandible is anteroposterior. The dentition differs from that of all the other Lemurs (and indeed from that of all the other Primates), and resembles that of the Rodents.

Thus there is only one pair of incisors in each jaw,(Among the Lemuridce, the outer and upper incisors of Nycticebus and Tardius soon fall out. Lichanotus and Tarsius have only one pair of incisors in the mandible.) and these grow from persistent pulps and have a thick layer of enamel on their anterior faces, whence they wear to sharp chisel-edges, like the incisors of the Rodentia. No canines are developed, and there are four grinders with simple crowns on each side above and below.

The formula of the milk dentition is d.i. 2.2/2.2 d.c. 1-1/0-0 d.m. 2.2/1.1.
The Lemuridae are confined to Eastern Asia, Madagascar, and South Africa; Madagascar presenting the greatest number and diversity of genera and species.


 
     
 
 
     



     
 
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