THE SLOTHS. 



181 



like the claws and nails of other mammals, 

 but envelop them all round. This important 

 distinction clearly separates the edentates 

 from the true clawed mammals, with which 

 they have often been associated. 



It is difficult to point out any other general 

 characters. The skeleton, the nature of the 

 skin and its appendages, the arrangement of 

 the limbs, and the structure of the internal 

 organs are all very different in the sloth, for 

 instance, on the one hand, and in the arma- 

 dillos and the ant-bears on the other hand. 

 The brain is always without convolutions, 

 and relatively to the size of the body and its 

 members must be called small ; the fore-limbs 

 are endowed with an extraordinary power of 

 rotation, especially in the sloths; the collar- 

 bone is always well developed; the pelvis 

 likewise peculiar in structure, containing alto- 

 gether a much greater number of vertebra 

 than in any other mammals. The move- 

 ments of these creatures are remarkably slow, 

 and this slowness may probably be attributed 

 to the great subdivision of the arteries, which 

 form the so-called " retia mirabilia" The 

 arteries, that is to say, are broken up at the 

 joints into a large number of ramifying or 

 anastomosing branches, which ultimately unite 

 again to form single vessels. The structure 

 of the placenta is so varied that nothing of a 

 general nature can be said about it. 



We subdivide this order into three fami- 

 lies: the Sloths, the Armadillos, and the 

 Ant-eaters. 



THE SLOTHS 



(BRADYPODA). 



The members of this family have a spherical 

 head and very long fore-limbs. The zygo- 

 matic arch is incomplete, and the jugal or 

 cheek bone has a downward process. The 

 tail is either altogether wanting or is a mere 

 stump. The body is covered with strong 

 bristles. The sloths are exclusively a South 

 American arboreal type. 



The upper part of the head has a swollen 

 or bulging appearance on account of the ex- 

 ceptionally large development of the frontal 

 sinuses, which extend back even to the oc- 

 cipital region, forming considerable cavities 

 between the plates of the bones of the upper 

 part of the skull. The jaws are rather short, 

 and the two halves of the high and strong 

 lower jaw are completely fused. The zygomatic 

 arch, only the front portion of which is de- 

 veloped, sends a long flat process downwards 

 so as to extend over the outer surface of the 

 lower jaw. The articulations between the head 

 and the neck vertebrae are so free that the 

 creatures are capable of turning the head 

 completely round to the back. As already 

 observed, the fore-limbs are extremely long; 

 thus, in the ai or three-toed sloth, which has 

 a total length of about 20 inches, the humerus 

 or bone of the upper arm measures about 6 

 inches, the two bones of the fore-arm each 

 about 4^4 inches, the hand or fore-foot from 

 the wrist to the end of the claws somewhat 

 more than 4 inches. The humerus is cylin- 

 drical and without any ridges for the attach- 

 ment of the muscles; the two bones of the 

 fore-arm are capable of executing much 

 greater movements of rotation than in man. 

 On account of this great freedom of rota- 

 tion and on account of the shape of the 

 head some naturalists have proposed to 

 associate the sloths with the monkeys. The 

 hind-limbs are thicker and shorter that those 

 in front; the thigh is cylindrical; there is 

 no third trochanter. In the ordinary attitude 

 of the animal the extremities show the em- 

 bryonic bend of the soles of the feet inwards ; 

 in walking the outer edge of the feet cor- 

 responding to the fifth digit touches the 

 ground. The two or three digits which are 

 present carry enormous claws flattened at 

 the sides and shaped like a sabre or sickle. 

 Frequently the metacarpal or metatarsal bones 

 of the other digits are also present. The 

 pelvis is very broad and flat, and the hip-bones 

 unite behind with the sacral vertebrae. The 



