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 Tattisia, Orycteropus, 

 Dasypiis, and others. It is the inilk 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. Bradypodidce — Sloths. — The three-toed sloths (Bradyfus) and the 



two-toed sloths (Clwlcepiis) 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 t)y 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 Cholapiis. The adult Bradypus has a distinct 

 coracoid or epicoracoid. 



As in most: herbivorous animals, the stomach is complex, but there is 

 no ciecum. In the limbs the main blood vessels break up into numerous 

 parallel branches. The uterus is simple, the vagina seems to be origin- 



44 



