Mylodon harlani, from Rock, Creek, Texas. 357 



The progressive dilatation of the neural canal to the middle 

 of the sacrum has also been remarked by Owen, who speaks of 

 its increase in size from a width of 2*5 inches and a height of 

 2 inches at the first lumbar in Mylodon rohustus. He does 

 not, however, record its maximum width. In the specimens 

 before me I can only record a width of 2*375 inches (5*5 mm ) 

 at the first lumbar to (circa) 4*5 inches (ll*4 mm ) at the second 

 sacral. The more perfect specimen, No. 10264, gives a height 

 of 2*625 inches (5*8 mm ). The height at the second sacral I can 

 not now record because of the degree of preparation of the 

 specimen. Posteriorly, the more perfect pelvis records a height 

 of 30*5 m,a and a breadth of 55*5 a ' m for the neural canal, showing 

 a very considerable decrease in its size at the seventh sacral. 

 The centrum of the seventh sacral, especially at its hinder end, 

 begins once more to assume a more typical form. The interver- 

 tebral foramina also exhibit a progressive increase and decrease 

 in size in correspondence with that of the neural canal. 



This gives abundant indication of the ample innervation of 

 the huge muscles of the pelvis, hind limbs, and tail, and one is 

 reminded irresistibly of the corresponding dilatation of the 

 neural canal in the Upper Jurassic dinosaur, Stegosaurus* It 

 is not possible to compare accurately the sacral content of the 

 two forms Stegosaurus and Mylodon, but the sloth sacrum 

 No. 10266 as preserved — approximately half of its original 

 length — would just about contain the sacral cast of Stegosaurus 

 (Cat. No. 1857), the paratype sacrum referred to S. ungulatus 

 and originally described by Marsh. The relative bulk of the 

 two animals was approximately two to one in favor of the 

 dinosaur, hence the vaunted sacral dilatation of the reptile is far 

 exceeded relatively by that of the mammal. In Stegosaurus 

 the brain weight was about one-twentieth of that of the sacral 

 mass. In Mylodon the brain case (see fig. 2) would seem to 

 show about one-third the capacity of the sacral canal (the 

 difference being largely one of length), showing a vastly greater 

 relative mentality over brutal reflex action in the Mylodon. In 

 the modern sloths Choloepus and Bradypus the brain is vastly 

 larger than the sacral cord, due perhaps not so much to an 

 increased cranial capacity as to a decrease in the relative mus- 

 cular development, especially of the loins and tail. Compare 

 fig. 15. 



The transverse processes of the first and second lumbars are 

 missing, but were irregularly rounded plates of bone. Those 

 of the third lumbar are fused with the expanded processes of 

 the sacrals and in turn with the inner margins of the ilia. 

 The separate elements are discernible only by the paired fora- 

 *Lull, this Journal, (4), vol. xxx, p. 372, fig. 9, 1910. 



