750 MAMMALIA. 



more than hints of enamel. Till recently the dentition was de- 

 scribed as monophyodont, but there is evidence of two sets in Tatusia, 

 Orycteropus, Dasypus, and others. It is the milk set which dis- 

 appears. 



A common primitive 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 Dasypus 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. Bradypodidse Sloths. The three-toed sloths (Bradypus) and the 



two-toed sloths (Cholcepus) 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 

 hang, 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 vertebrae in Brady pus > 

 of six in Cholcepus (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 caecum. 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. Megatheriidae 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. 

 Megatherium exceeded the rhinoceros in size. Near the Mega- 

 theriidae the recently exterminated or still living Neomylodon 

 may be included. 



