750 MAMMALIA. 
more than hints of enamel. Till recently the dentition was de- 
scribed as monophyodont, but there is evidence of two sets in 7a¢usia, 
Orycteropus, Dasypus, and others. It is the milk set which dis- 
appears. 
A common frimitive character is the persistence of the testes in the 
abdominal cavity. 
The placenta shows much diversity, but the reproductive phenomena 
are imperfectly known. In the sloths and ant-eaters the placenta is, 
usually described as dome-shaped ; but according to some authorities 
this is merely a stage in the growth of a placenta, which is at first poly- 
cotyledonary, and later discoidal. The discoidal deciduate type appears 
again in the armadillos, but in Dasyfus among them it is said to be 
zonary. In the pangolins it is diffuse and indeciduate ; in Orycteropus, 
apparently by a suppression of the polar villi of a diffuse type, it is 
zonary, and doubtfully deciduate. 
Order XENARTHRA 
1. Bradypodidaee—Sloths.—The three-toed sloths (Bradypus) and the 
two-toed sloths (Cholwpzs) are restricted to the forests of S. 
and Central America, They are the most arboreal of mammals, 
passing their whole life among the branches, to which they 
bang, and along which they move back downwards. They are 
solitary, nocturnal, vegetarian animals, sluggish, as their name 
suggests, and with a very firm grip of life. Their shaggy hides 
harmonise with the mosses and lichens on the branches, and 
the protective resemblance is increased by the presence of a 
green alga on the hair. Their food consists of leaves and 
shoots and fruits. 
The body is covered with coarse shaggy hair; the head is rounded, 
and bears very small external ears; the fore-limbs are longer than the 
hind-limbs, and the two or three digits are bound together by skin, and 
have long claws; the tail is rudimentary. 
Concerning the skeleton we may note the rootless, unenamelled, 
peg-like teeth, the incomplete zygomatic arch with a descending process 
from the jugal, the presence of clavicles, the rod-like appearance of the 
embryonic stapes, the occurrence of nine cervical vertebra in Bradypus, 
of six in Cholepus (but see p. 694). The adult Bradypus has some- 
times a separate coracoid or epicoracoid. 
As in most herbivorous animals, the stomach is complex, but there is 
no ceecum. In the limbs the main blood vessels break up into numerous 
parallel branches. The uterus is simple; the vagina seems to be 
originally divided by a median partition; the placenta is deciduate, 
and changes in shape during development. One young one is born at 
a time. 
2. Megatheriidee or Ground Sloths—extinct forms of large size, 
intermediate between the sloths and the ant-eaters. Their 
remains are found in Pleistocene deposits in N. and S. America. 
Megathertum exceeded the rhinoceros in size. Near the Mega- 
theriidze the recently exterminated or still living Meomylodon 
may be included. 
