44 



prolonged, but is less deeply channelled above than in the Megatherium, and is not 

 so distinctly denned by the abrupt increase of depth of the ramus behind it which 

 characterizes the Megatherium ; the molar part of the mandible makes, however, a 

 greater convexity below than in the Mylodon. 



With these marks of approximation to the Megatherium there are, however, the 

 same differences as in the Mylodon in regard to the widely open orbit, the more simple, 

 trifurcate, malar bone, the minor depth of the alveolar portions of the jaws, and the 

 straighter outline of the lower border of the mandible. In both the Mylodon and 

 Scelidothere the coronoid process is relatively shorter than in the Megathere, and 

 the foramen near the fore-part of the base of that process is outside and below that 

 base, not on the inside of it as in the Megatherium. The mastoid process is relatively 

 shorter and the stylohyal pit is shallower; the lacrymal bone is more distinct, and 

 the foramen is larger in the Scelidothere than in the Mylodon or Megatherium. In 

 all the essential characters of the lower jaw, as in the number, structure and kind of 

 teeth, the extinct megatherioid quadrupeds more closely resemble each other, and the 

 existing Sloths, than any other known existing or extinct animals. 



The number of the teeth, their length, equable breadth and thickness, and absence 

 of fangs, their deeply excavated base, and the unlimited growth resulting from the per- 

 sistent matrix, together with their composition of cement, dentine and vaso-dentine, 

 without any true enamel, are characters common to the Megatherioids and Sloths. 

 The form of the teeth differs in, and characterizes, each genus. It would seem that 

 the Megalonyx, in the elliptical or subcylindrical shape of such of its teeth as are 

 known, more closely resembled the existing Sloths than do the other Megatherioids. 

 The Unau (Cholaepus didactylus) resembles the Mylodon in the distance between the 

 first and the second molars of the upper jaw; but the advanced molar assumes, in 

 that existing Sloth, the form and proportions as well as the position of a canine, and 

 the corresponding tooth of the lower jaw is similarly developed and separated from 

 the other three teeth by a nearly equal interval. In the Ai (Bradypus tridactylus) 

 the first molar in both jaws presents nearly the same proportionate size to the rest, as 

 in the Megatherium, and is not separated from them by a wider interval than the 

 rest, as it is in the Unau and Mylodon. In both species of existing Sloth, the last 

 molar of the lower jaw is as simple in form as in the Megatherium : in the Mylodon 

 and Scelidotherium it is larger and more complex in shape, the grinding surface 

 being divided into two lobes by two oblique channels, which traverse longitudinally 

 one the outer the other the inner side of the tooth : these grooves are more shallow 

 in the Scelidothere than in the Mylodon, and the lobes of the tooth are more equal 

 and more compressed. The grinding surface itself, in all the molars of both Mylo- 

 don and Scelidothere, resembles that in the Sloths; the two transverse ridges are 

 developed only in the teeth of the Megatherium, which are longer in proportion to 

 their thickness than in the Mylodon, Megalonyx, or Scelidothere. These modifica- 

 tions, with the narrow palate, the close-set series of teeth, their great length, and the 



