FAMILIES OF EDENTATA. 689 



part of the mouth, and they have not more than hints of enamel. Till 

 recently the dentition was described as monophyodont, but we now 

 know that there is evidence of two sets in Tatusia, Orycteropus, 

 Dasypus, and others. It is the milk set which disappears. 



The modern Edentata are specialised survivors of a wan- 

 ing order, whose extinct representatives seem to have been 

 larger and more primitive. The modern forms usually have 

 protective peculiarities of structure and habit which secure 

 their persistence. Thus some are arboreal, others are bur- 

 rowers, and many are covered with strong armature of bone 

 or of horn. It is interesting to observe how very varied the 

 nature of the placenta is : 



a dome-shaped disc (deciduate) in the sloths, 

 dome-shaped or discoidal (deciduate) in the ant 



eaters, 



discoidal (deciduate) in the armadillos, 

 diffuse (non-deciduate) in the pangolins, 

 zonary (deciduate) in the aard-varks. 



Families of Edentata. 



i. Bradypodidae Sloths. The three-toed sloths (Bradypus) and the 

 two-toed sloths (Chol&pus] are restricted to the forests of South 

 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 



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 

 Bradypus, of six in Cholcepus. The adult Bradypus has a distinct 

 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 origin- 

 44 



