SLOTHS. 
101 
In old animals most of tlie bones ol the wrist and ankle joints 
become united. In addition to the difference in the number of 
their claws, the Three-toed Sloths, or Unaus, and the Ais, or 
Two-toed Sloths, are distinguished by the different shape and 
proportions of their teeth, — those of the former being small, of 
equal size^ and the upper ones placed opposite the lower, so 
that they wear down nearly flat ; while in the latter the first 
tooth in each jaw forms a sort of canine, twice as long as any 
of the others, and as the teeth are placed alternately in the two 
jaws, they are worn down into wedge-shaped crowns. Skeletons 
and skulls of both genera are placed in the case. 
Intermediate to a certain extent between Sloths and Ant- 
eaters are certain huge fossil animals, found in the Tertiary 
deposits of South America, of which the best known is the 
Great Ground-Sloth [Megatherium americanum^ 1385 ) ; a cast 
of the skeleton is exhibited in the Palfeontological Gallery, 
and one tooth in case 32 of the Lower Mammal Gallery. 
Mylodon is an allied genus. In the earlier Tertiaries of 
Patagonia the species are much smaller : one of the larger 
kinds certainly existed within the human period. 
The Anteaters [Myrmecophaguke) have narrow heads with [Case 32. 
long snouts, to accommodate their enormously elongated, worm- 
like tongues ; their tails are well developed, and in some species 
prehensile, their toes are separate from each other, as in ordinary 
Mammals, and the third on the fore-foot is provided with a 
huge digging claw. Like Sloths, they are natives of Tropical 
America. The Great Anteater, Myrmecopliaga juhata ( 1388 ), 
is about four feet in length and has a long black mane along 
its back, a thick bushy tail, and a long narrow head, about 
a foot in length, the greater part of which is made up by the 
maxillary bones. There are no zygomatic, or cheek, arches to 
the skull, little biting-power being needed ; and the collar- 
bones, or clavicles, are exceedingly rudimentary. This Anteater 
is terrestrial in its habits, and feeds entirely on ants, which it 
catches with its long sticky tongue, after having torn open 
their nests with its powerful claws. Much smaller are the 
Tamandua, Tamar^dua tridactyla ( 1387 ) and the Two-toed 
Anteater, Cycloturus, or Cyclopes, didactylus ( 1386 ), the 
