PREVIOUS TO THE LAST GEOLOGICAL REVOLUTION. 157 



of the great Megatherium on the spot, considered the animal 

 described by Jefferson as specifically identical. 



The numerous remains which several species of this genus 

 have left in the caves of Brazil, enable me not only to eluci- 

 date this point in science, but also to throw some light on the 

 habits of this very remarkable animal, so different from any 

 now living. 



The Megalonyx forms a genus perfectly distinct from the 

 Megatherium. It resembles, nay exceeds, the latter, in the 

 thick clumsy shape of its body. The construction of the 

 fore and hind feet is the same ; but in the formation of its 

 head, and particularly in the number, shape, and position of 

 its teeth, it differs greatly. The Megalonyx has five molars 

 in the upper, and four in the lower jaw. The teeth are flat- 

 tened cylinders, somewhat curved, both in a longitudinal 

 and transverse direction, and inserted obliquely in the jaws : 

 they present a rather excavated grinding surface, the margin 

 of which is notched at one end. The posterior molar of the 

 under jaw has a somewhat consolidated aspect, as if it were 

 formed by the union of two. Like the Megatherium, it had 

 a long and strong tail, composed of many vertebrae, and ex- 

 traordinarily powerful, especially at its root. Its ribs are 

 grooved longitudinally along its external surface ; a character 

 only observed in some species of Dasypus. However, as it 

 is not my object to give here a detailed description of this 

 animal, I will confine myself to the general observation, that 

 most of the points in which Megalonyx differs from Mega- 

 therium, present so many approximations to the modern 

 sloth ; one of which is of too great importance to be passed 

 over without examination. It is well known that in the three- 

 toed sloth, the foot articulates with the tibia, not, as in all 

 other Mammalia, in a plane at right angles to the leg, but on 

 a plane continuous with it ; so that if this creature wished to 

 stand upright upon its four legs, it would have to rest upon the 

 outer edge of the soles of its feet; while, in order to rest 

 upon the flat of the sole, it must lie on its belly, with its feet 

 stretched out straight from its body. This peculiarity, of 

 which we are as yet acquainted with only this one living ex- 

 ample, is repeated in the Megalonyx, although the mechanism 

 whereby this distortion, if we may so call it, is effected, dif- 

 fers much in the two animals. In the sloth it is produced 

 by the singular mode in which the tibia andjibula articulate 

 with the astralagus ; while in the Megalonyx this joint occurs 

 in the manner usual among Mammalia ; and the irregularity 

 of the plane of the foot depends on the articulation of the first 

 row of the metatarsal bones with the astralagus and calca- 



