MAMMALTA, 533 
CHAPTER XXIX. 
6. SLOTH. 7. MOLE. 8. PORPOISE. 9. BAT. 
VI.—THE StotH (Bradypus tridactylus) — Arboreal. 
The sloths are truly arboreal mammals, being very com- 
pletely adapted to a tree life. The hair is shaggy and 
its neutral tint is much in harmony with its surroundings, 
the more so in those species which cultivate the growth of a 
green alga upon the hair. The external ears are reduced, 
probably for easy passage through boughs, and the tail is 
vestigial. The fore-limbs are longer than the hind, as one 
tendency of the arboreal habit is the greater use of the fore- 
limbs. The sloths are herbivorous, feeding solely upon the 
leaves of trees, and they belong to one of the lowest orders 
of the Lutheria, namely, the Edentata. We shall therefore 
expect to find in them certain “ edentate” characters, others 
due to a herbivorous diet, and yet others correlated with the 
arboreal habit. 
The skull has several peculiarities. The zygomatic arch 
is not complete, as the jugal does not reach back to the 
squamosal, and the premaxille -are nearly absent, which 
assists in reducing the facial part of the skull in proportion 
to the cranial. The incisor teeth (on premaxilla and oppo- 
site it) are absent, and so most probably are the canines 
(though in the two-toed sloth the first tooth is long and 
pointed like a canine). There are five stump-like homodont 
teeth in the upper jaw and four in the lower jaw. These 
grow from persistent pulps, as they are worn away and have 
no enamel. An outer layer of cement envelops the hard 
thin coat of dentine, which in its turn encloses a softer vaso- 
dentine. In use the same principle is involved as in other 
herbivorous types, the hard dentine here playing the part of 
enamel and forming the slowly-wearing ridges between cement 
and vasodentine. So far as is known there is no milk 
dentition. It is difficult to say how far these peculiar dental 
characters are due to degeneration from a higher Eutherian 
type, or how far they are due to a primitive condition. 
