Mylodon (Gr. mill, and, tooth), a genus of gigantic fossil edentates established by Prof. Owen, and closely allied to the sloth, resembling megalonyx and megatherium. The mylodon has the heavy form of the megatherium, with a dentition resembling that of the megalonyx; the molars are 5/4 - 5/4, and are worn into flat surfaces; in the upper jaw, the first is subelliptical and separated from the rest, the second elliptical, and the others triangular, with the internal surface grooved; in the lower jaw, the first is elliptical, the third quadrangular, and the last the largest and bilobed, and the symphysis stronger than in the megalonyx. The head resembles that of the megatherium in its form, and has a strong descending process of the zygomatic arch; the extremities are equal, the anterior five-toed, and the posterior four-toed; the two external fingers are without nails, and the others have large semi-conical and unequal claws; the acromion and coracoid are united, the radius turns around the ulna, thetibia and fibula are distinct, the heel bone long and large as in the other megatherioids. (See Megalonyx, and Megatherium.) The M. Darwinii (Owen) was discovered by Mr. Darwin in northern Patagonia; the symphysis of the lower jaw is long and narrow, with the second molar subelliptical, and the last with two furrows, of which the internal is angular; it is found from the pampas of Brazil southward.
The M. Harlani (O wen) has the symphysis shorter and wider, the second molar square, and the last with three grooves, the internal one biangular; this has been found in Kentucky, Mississippi, Missouri, South Carolina, and Oregon. The M. rohustus (Owen) is characterized by a short and wide symphysis, with the second molar subtriangular, and the last with three grooves, of which the internal is rounded. A fine and nearly complete skeleton of this species is now in the museum of the London college of surgeons; it was discovered in 1841 in the fluviatile deposits about 20 m. north of Buenos Ayres, recently elevated above the level of the sea. The skeleton is very robust; the trunk, shorter than that of the hippopotamus, ends in a pelvis as wide as and deeper than that of the elephant; the hind limbs short and massive, with feet as long as the thigh bones, set at right angles to the leg, and with the sole turned slightly inward; the tail as long as the hind limbs, very thick, and affording a firm support in the semi-erect position; the chest long and large, protected by 16 pairs of ribs, broad and strongly attached to a well developed sternum; the scapula) unusually broad; arm bones thick and short, with strong processes for muscles; the bones of the forearm longer than those of the leg; the skull smaller than that of the ox, but long, narrow, with a truncated muzzle, and supported by a short neck of seven vertebrae; dorsal vertebrae 16, with broad and high spinous processes nearly equal and having a uniform backward inclination.
Such proportions are found in no living animals, and only in the megathe-rioids among fossils. The skull presented two extensive fractures, from which the animal had recovered; the air cells extend from the frontal and ethmoidal sinuses into the cranial bones, separating the two tables of the skull sometimes for the extent of 2 in., forming a great protection against injury from falling limbs of trees. They were probably peaceful animals like the existing sloths, though able to inflict severe wounds by their sharp and heavy claws; the muscular strength of the edentates is very great, and must have been immense in all the megatherioids. While presenting the closest affinity to the small arboreal sloths, the mylodon, with its claw-armed inner toes, had the outer thick and stunted, and evidently enveloped in a kind of hoof, giving the power of standing; and walking firmly as well as digging and seizing - in this respect marking a transition between edentates and pachyderms. It is now generally admitted that this animal commenced the process of prostrating trees by scratching away the soil from their roots, and loosening them from their attachments; then, seizing the branches or trunk, and supported on the hind limbs and tail, it swayed the tree to and fro, and soon brought it to the ground to be stripped at its leisure; in case of meeting a tree too large to be uprooted, it is probable that some of the smaller species, as indicated by the inward turning of the soles, possessed the faculty of climbing to the larger branches within reach of the foliage.
In regard to the means of stripping off leaves, Prof. Owen, from the cavity in the mastoid process for the articulation of the hyoid bone, and the large size of the anterior condyloid foramina whence issue the motor nerves, maintains that there was a remarkable development of the tongue; this is also indicated by the broad, smooth, concave surface of the symphysis of the lower jaw, which, with the absence of incisors, offered no obstacle to its free motions, and provided space for it when retracted; the megatherium had a short proboscis, prehensile lips, and a smaller tongue in a narrower mouth; the elephant has a maximum proboscis, the giraffe a maximum tongue, the megatherium being intermediate; the mylodon, having no proboscis, had a largely developed tongue for stripping off foliage, contrasting in this respect with the almost tongue-less elephant. While the megatherium may. have measured 18 ft. from the fore part of the skull to the end of the tail, following the curve of the spine, the mylodon measured only 11 ft.; other measurements in these animals respectively were: circumference at pelvis 141/3 and 2/3f ft.; length of skull 22/3 and 1½ ft,, greatest width 1½ and 1/1 ½ ft.; length of lower jaw 25½ and 15½ in., width at symphysis 5 2/3 and 51/3 in.; length of anterior limb 10 and 4½ ft.; clavicle 15 and 81/63 in., humerus 2¼ and l1/3 ft., ulna 25½ and 14½ in., radius 26 and 11 in.; fore foot 31¼ and 14 in. long, and 14¼ and 81/3- wide; middle and longest claw 10½ and 5¼ in.; width of pelvis 61 and 41 in.; length of femur 28½ and 19 in., circumference over great trochanter 31/6 and 21/6 ft., and width at same point 16 and 9 in.; tibia 22 and 8½- in.; length of hind foot 34½ and 19 in., width 12 and 6|; heel bone 17 and 7½ in.; middle and largest claw 9½ and 51/3 in.; and width of largest vertebra of tail 21 and 10½- in. - The scelidotherium (Gr., hind leg, and, animal) is another extinct me-gatherioid, remarkable for the size of the hind limbs; a nearly entire cranium shows the essential characters of the sloth's skull, with the mylodontal modifications of the complete zygoma and shape of the lower jaw; the teeth were 5/4 - 5/4, the upper triangular; the form was massive.
Pictet mentions seven species, varying in size from a hog to an ox, which lived in South America during the diluvial epoch. Some other genera have been described by Owen, Pictet, and Leidy.
Mylodon robustus (restored).