12 VERTEBRATA. 



and an equal number of sacral vertebrae. The part in which the Megatherium 

 least resembles the Sloth, is the tail ; and, as a general rule, in those modifica- 

 tions of structure in which it differs from its living analogue, it approximates to 

 the Anteater ; e. g. in the number and structure of the true vertebrae. 



The astragalus is the most characteristic single bone in the skeleton ; its 

 upper surface being so hollowed on one side, with a wide crescentic groove, as to 

 throw the whole weight of the leg upon the inner side of the foot. The anchy- 

 losis of the tibia and fibula is known among existing quadrupeds only in armor- 

 bearing Edentates. 



The head of the Megatherium is remarkable for its relatively small size, for 

 the extraordinary depth of the lower jaw, and for the great size of the zygomatic 

 processes. The length of the skull is thirty inches : three inches less than that 

 of the Asiatic Elephant. The formation of the muzzle indicates the possession 

 of a short proboscis. The dental formula is 5__5_ — - 18. The teeth are pris- 

 matic and slightly curved, and measure from eight to ten inches in length, and 

 between two and three inches in breadth, In composition, a central axis of vas- 

 cular dentine is inclosed by a wall of unvascular dentine, and this by one of 

 cement. 



The spinal column consists of seven cervical, sixteen dorsal, three lumbar, 

 five sacral and eighteen caudal vertebrae, and measures fifteen feet in length, or 

 three feet more than the Elephant's. The circumference of the skeleton, at the 

 eighth rib, is eleven feet. 



The Megatherium differs strikingly from existing quadrupeds of corres- 

 ponding bulk, in the vast proportions of its anterior extremities. Its clavi- 

 cle, fifteen inches long, is the longest known. The foreleg bespeaks enormous 

 strength; with the foot, it is seven feet and four inches in length. . 



The posterior extremities are shorter than the anterior. The pelvis is the 

 largest bone in any land animal, living or extinct ; it is upwards of five feet 

 broad. The rugged ilium and spinal crest show that it was the centre of muscu- 

 lar bundles of enormous power, which diverged to act upon the trunk, the tail 

 and the hindlegs. These muscles, judging from the size of the spinal cord, 

 which in this region is four inches in diameter, must have been characterized by 

 the extreme energy of their vital contractibility. The acetabulum is excavated 

 in a very exceptional manner, its concavity facing directly downward. This gave 

 increased strength for sustaining vertical pressure at the expense of rapid motion. 



The hindlegs appear more like columns for support than organs for locomo- 

 tion, and, with the hindfeet, are models of massive organic masonry. The heel 

 bone alone has the extraordinary length of seventeen inches, and a circumference 

 of twenty-eight inches. The monster walked, like the Anteater, on the outside 

 edge of its foot, on a marginal hoof-like callosity. The middle toe of the hind- 

 foot, and likewise the second, third and fourth digits of the forefoot, were armed 

 with powerful claws. The magnitude of the tail fills the observer with wonder : 

 when clothed with flesh, it must have been more than six feet around at the 

 greater end. With the hindlegs, it formed a tripod upon which the animal rest- 

 ed when obtaining its food. 



It would be interesting to know something of the daily life of the animal 

 whose colossal size was united to such strange anatomy. As the brain of the 

 Megatherium was less by nearly one-half than that of the Elephant, we infer 

 that he was a creature of fewer instincts. Nevertheless, his cotemporary quad- 



