SLOTHS. 97 



Order X. EDENTATA. 



The Edentata as a whole are characterized by the incomplete [Case 23.] 

 state of their dentition. In many forms teeth are entirely absent, 

 while when present they are always composed of dentine and 

 cement only (without enamel), and never form roots. In only one 

 genus (Tatusia) is there a functional milk -dentition, one only 

 (Dasypus) possesses premaxillary teeth, and in none is there any 

 definite division of those in the maxilla into canines, premolars, and 

 molars. 



The first family, the Bradypodidae, or Sloths, have short round 

 heads, five teeth above and four below. The vertebrae, owing to 

 the inactive habits of Sloths, are almost entirely devoid of neural 

 spines, and the whole vertebral column, especially in the neck 

 region, is remarkably flexible and loosely united. The cervical 

 vertebras, which in all other Mammals, except the Manatee, are 7 

 in number, amount to no less than 9 in the Three- toed Sloths 

 (Bradypus)) while in certain of the Two-toed Sloths (Cholcepus) 

 there are only 6. In Cholcepus the clavicle is attached to the cora- 

 coid, a peculiarity not occurring in any other Mammal. The pelvis 

 is remarkable for being united to an unusually long portion of the 

 spinal column, both the ilium and ischium being attached by bone 

 to the latter. In old animals most of the carpal and tarsal bones 

 ankylose together. In addition to the difference in the number 

 of their claws, the two genera Bradypus and Cholcepus are also 

 readily distinguished by the different shape and proportions of their 

 teeth, those of the former being small, of equal size, and the upper 

 ones placed opposite the lower, so that they wear down nearly flat ; 

 while in Cholcepus the first tooth in each jaw forms a sort of canine, 

 twice as long as any of the others, and as the teeth are placed 

 alternately in the two jaws, they are worn down into wedge-shaped 

 crowns. 



Skeletons and skulls of both Bradypus and Cholcepus are placed 

 in Case 23, Div. E. 



Intermediate to a certain extent between the Sloths and Ant- 

 eaters are certain huge fossil animals, found in the post-Tertiary 

 deposits of South America, of which the best known is the Great 

 Ground-Sloth (Megatherium americanum), of the complete skeleton 

 of which a cast is exhibited in the Geological Gallery. 



