OSTEOLOGY. —If we take a general survey of these eight genera of monkeys of the New World, we may observe in them, as well as in those of the Old World, an indication of the descending line, by which they pass into the form of the Lenzurince, and by those into the Insectivora. In this way they constitute a series, which is. parallel to that of the monkeys of the Old World, the latter passing into the Carnivora, the former into the Insectivora. The truth of this assertion will be proved by a more minute examination of the skeleton.
We shall first consider the skull. J. A. WAGNER divided the monkeys of the New World by their skull into two great divisions. The first is a pyramidal skull, in which the height is greater than the length, and in which the occiput has no posterior eminence, and the occipital forainen is situated backwards. To this division belong Illycetes, as eminently characteristic, and, in subsequent gradation, Nochthora, Pithecia, and Lagothri.r. The second form of skull is elongated, with a prominent muzzle, a convex occiput, and an occipital foramen, situated at the basis of the skull. WAGNER refers to it the Saimiri, offer ing a typical pre-eminence, and subsequently Hapale, Cebus, and Aides.
In Mycetes (fig. 132) the forehead is ele vated, the face flat and large ; the distance between the two orbits very great; two nasal bones ; the chin very depressed ; the lower jaw high, with distant branches, between which the inflated hyoid bone is situated. The same character is to be found in the genus Ateles. In Illyeetes, Lagothrix, and Callithrix, there is a peculiar round aperture in the orbital portion of the zygomatic bone, which has the appearance as if it were pierced in the bone by a gimlet. Mycetes, Aides, and especially Callithrix, afford a very striking conformity with Hylobates, in the swollen ap pearance of the posterior wall of the orbits, produced by the convexity of the orbital part of the zygomatic bone. This is a new addi tion to the analogy between Hylobates and Aides. The ala magna ossis sphcenoidei is yet more depressed backwards than in Hylobates. In Cebus (fig. 133) the cranium is elongated, and uniformly round. The frontal bone is lengthened to a sharp point, which advances between the two parietals. This is, as I have said before, a manifest indication of a lower rank. The face is not very promirient ; there are two nasal bones ; a distinct intermaxillary bone ; a rounded chin, which recedes. In Callithrix, Pithecia, and Nochthora, the skull has an oblong form, but it resembles very much a small human skull. The single frontal bone has a triangular form, and is distin guished by the convexity of the orbital part. ln the Saimiri the septum between the orbits is but membranous, and the interorbital space narrow ; the nasal bone is sometimes single, sometimes double ; the intermaxillary bone distinct ; the chin round and prominent ; the muzzle not protruding ; the orbital part of the zygomatic bone wants the opening proper to Aides, Mycetes, Lagothrix, and the other species of Callithrix. This general re semblance to the human skull is still greater in the Ouistitis. The external tuberosity of the orbit is less marked; the interorbital sep tum is osseous ; the muzzle not very promi nent; the intermaxillary bone distinct, but obliterated in old age ; the nasal bones broad, short, completely separated, and consequently similar to those of man ; the chin is depressed, but rounded. Notwithstanding this general resemblance to the skull of man, Cebits, Calli thrix, and Hapale differ in some essential points from man. The forehead is much narrower, and has its greatest elevation not laterally, but in the middle; the occipital fora men placed more backwards ; the muzzle more protruding. In the vertebral column of all the Cebince there is a manifest inferiority to be seen in the disposition of the cervical ver tebrw, in which there are anterior ridges at the transverse processes, in the same manner as in the lower Mammalia. In the Cebi, the spinal process of the second cervical vertebra offers another analogy with the latter, in its elevated form, in its strength, and in its truncated posterior edge. In the Sainziri the tendency to a lower degree of pefection is still greater, by the triangular form of the transverse processes, and in the Ouistitis the spinal processes become long, acute, and di rected backwards. The number of dorsal vertebrw varies from 13 to 14, and is con sequently in general greater than in the monkeys of the Old World. There is oppo
site direction between the spinal processes of the three last, and the ten or eleven first dorsal vertebrw. The same disposition is observed in the Saimiri, but in the Ouistitis there is only opposition in the spinal pro cess of the last dorsal vertebra. In Ateles and Cebus the number of lumbar vertebrw is five. The styloid processes are plainly indicated, but their spinal processes are inclined for wards, and terminate in a recurved point, in the same manner as in the Carnivora. In the Ouistitis the analogy with the quadru ped form is still greater, as the styloid pro cesses are very long. In the Nochtlzora the number of the lumbar vertebrw is eight, by which it approaches to Stenops. The sacrum is in the Cebince a broad quadrangular bone, with acute edges, united only by one of its spu rious vertebrm with the iliac bones. Conse quently the synzphysis sacro-iliaca is less firm than in the higher species of monkeys. At least such is the case in the Cebi, the Ouistitis, and the Saimiri ; but in Ateles I found four spurious sacral vertebrx united with the iliac bones. The iliac bones are in general nar rower in the Cebince than in the monkeys of the Old World : consequently the pelvis has a more cylindrical form, with a very long pubic articulation, and approaches more to the form of the pelvis in the Carnivora. The caudal vertebrce of the Cebinte deserve a se parate mention. They are true or spurious ver tebrze. The true are but four or five, short and thick. The spurious are the longest, but become shorter at the extremity of the tail. They are only united by the bodies, not by the articular processes. Chiefly remarkable are the inferior spinal processes in the an terior caudal vertebrce, representing the letter V, and forming a canal, in which pass the vessels for the tail. These processes disap pear in general in the posterior caudal ver tebrx, and in the monkeys with a prehensile tail the posterior vertebrw become round, tubercular bones, imitating a series of small digital phalanges. The thorax of the Cebince is compressed, and the ribs do not form the posterior arches, by which the back of man, of the Chimpanzee, and of the Orang-cetan acquire a broad and flat surface, and by which it is possible for these animals and for man to lie at full length on their back. All the species, on the contrary, which possess ischial callosities, the Gibbons among the rest, sleep and repose themselves in a sitting posture, with the arms folded across the knees, and the head reclined upon the breast, or sup ported by the shoulder. The Cebince, which the ischial callosities are wanting, lie down on the lateral surface of their body. The sternum is separated in the Cebince into as many segments as there are true ribs ; consequently it has quite lost the analogy with the human subject, which it has in the higher monkeys of the Old World. In the anterior extremities, the humerus of Cebus, Nochthora, Saimiri, and Ouistiti, is similar to that of the Carnivore, by an aperture in the internal condyle, serving for the passage of the brachial artery and the median nerve, which are preserved in this manner from compression and injury, by the contraction of the muscles in the climbing motion of these Quadrumana. In the carpus of the Cebinc there are nine bones, and consequently they possess the intermediate bone, proper, as I have said, to all the monkeys, with the ex ception of the Chimpanzee. The phalanges of the fingers and the toes are in general very long and incurvated, by which disposition they acquire a greater aptitude to grasp branches of trees, while climbing. In Ateles the fore-hand has quite lost its analogy with the hand of man, by the want of the thumb, which is only represented by an imperfect metacarpal bone. In Ateleshypoxanthus, which has a rudimental thumb, Prince MAximiLiAN says that it consists of two phalanges, of which the first is but half as long as the second. In the Cebi, the fore-hand differs from the hand of man, by the deviation of the thumb, which is situated on the same level as the other fingers, and has the same length as the little finger. The nails are elongated, and acquire really the form of little claws in the Ouistitis. The posterior extremities offer the general character of the posterior extremities in the monkeys ; the thumb of the hind hand is distant, and has a flat nail in the Ouistitis, while on the other fingers there are small claws.