ANATOMY OF THE SLOTHS. 



163 



muscles, with some important differences, which result in there being a greater amount of bending 

 and extending of the foot, although the foot rests 011 its outer edge. 



A Sloth's face is short, and there is a broad snub nose, with round nostrils, which are widely 

 open. The cheeks become wide suddenly, and the forehead slopes rapidly backwards, the eyes being 

 wide apart and small, but looking forwards. The head is small and round, and as it is covered 

 with hair behind, it cannot be distinguished well from the upper part of the back of the neck. The 

 expression of the face is always the same, and the method of masticating and eating is disagreeable 

 to observe. The animal having no front teeth, and moving its jaws usually only upwards and down- 

 wards, and not from side to side, places the morsel, such as lettuce leaf or carrot, well into its 

 mouth, and chews at it, dragging out the food every now and then, when it is covered with moisture. 

 On examining the skull, the short cut off or truncated appearance of the face is very evident, and it will 

 be observed that the teeth are wanting in the front bones of the face (the pre-maxillaries), and that only 

 the palatal part of these bones exists. The lower jaw is strongly jointed to the upper, and the back part 

 is large : there are teeth at the sides, but there are none in the front part of it. A very singular-looking 

 cheek bone (zygoma) exists on either side. It is not attached behind to the ear bone, so as to cover 

 the jaw muscles, but it has two processes behind 

 an upper and a lower which differ in shape 

 and size according to the species. The central 

 bone of the nose does not reach to the nasal out- 

 let, and there is a system of air-cavities which is 

 continued from the nose into the forehead bone. 

 In some kinds, the lower jaw ends abruptly in 

 front, as in the Ai ; but in the Unau Sloths it 

 is slightly angular, and projects. 



The back teeth of the Sloths are very simple, 

 and consist of three structures, oalled vaso -dentine, 

 hard dentine, and cement, there being no proper 

 namel. The vase-dentine is a kind of bony sub- 

 stance in the centre of the tooth, in which there 

 are the passages and tubes of blood-vessels. The 

 dentine is outside this, and consists of more earthy 

 particles than the vaso-dentine, and of fewer 

 tubes ; it is all the denser and more resistant. 

 Wearing away more slowly than the vaso-dentine, 

 it forms a ridge which grinds easily. The cement is a kind of bony structure on the outside of the 

 tooth. The teeth of the Sloth continue to grow from below as they are worn above, and there is no 

 entire milk set which are replaced by those of a permanent kind. 



The term Sloth is commonly applied to all the kinds of animals whose general shape and habits 

 have just been noticed. It is evident, however, that this union of several species under one term is 

 not correct in zoology, and it is necessary to distinguish them by peculiarities which are permanent. 

 A very ready method of distinction is to separate the Sloths into two families, one containing 

 those which have three claws on the fore limbs and the same number on the hind limbs, and the 

 second including those which have only two claws on the fore feet and three on the hinder. 



The first family is called the BRADYPODID./E, from ppadvs (slow), and irovs (foot), and the second 

 OHOLCEPODID^E, from x u *6* (halting, lame), and irovs (foot), and both are included in the group 

 TARDIGRADA, or slow-moving JZdentata. 



The BRADYPODIDJE include two genera, but many naturalists only acknowledge one. The first 

 is Bradypus. This includes the Sloths with three-clawed fingers on the fore limbs, whose males and 

 females are alike in their fur, and which have the cheek bone (malar bone) with two processes. 

 The upper one is long and dilated at the end, and the lower is long and triangular, and neither of the 

 processes reaches the ear bone. There are in these Sloths, when full grown, five molar teeth on each 

 side in both jaws, and the first is very short. There are two mammte on the chest. 



* Arctopithecus castaniceps. 



SKULL OP SLOTH.* 



(From the Proceedings of the Zoological Society.) 



