1917] 



Stock: Skull Structure of Mylodont Sloths 



167 



men showing the greatest constriction is no. 646, while nos. 642, 643, 

 and 695 show successively less constriction. The Nebraska skull de- 

 scribed by Barnum Brown 4 is slightly more constricted than no. 646, 

 but the actual difference is so small that it is negligible. In this 

 arrangement, the skulls, including the Nebraska specimen, exhibit a 

 variation in length of approximately only 30 mm. 



In a former paper the writer 5 stated that the inflation of the 

 muzzle in the mylodont skulls of the University of California series 

 was similar to that in the Nebraska skull. This is true also for the 

 greater number of skulls contained in the Museum of History, Science 

 and Art of Los Angeles, as far as the actual diameter of muzzle is 

 concerned. In proportion to the postorbital constriction, however, 

 many of the skulls, including the slender types, are slightly less 

 inflated at the muzzle than Brown's specimen. 



The palate posterior to the fifth superior tooth is very narrow in 

 the slender type of skull from Ranch o La Brea. This character 

 apparently does not depend on the slenderness of the cranium, since 

 in M. garmani the palatal index is actually greater than in a broad 

 skull of M. harlani (no. 21158 Univ. Calif. Coll. Palae.) of approx- 

 imately the same length. Furthermore, in no. 21160 Univ. Calif. Coll. 

 Palae., a narrow palate, which is 57.5 mm. in width, is associated with 

 a broad cranium. The slender skulls again lend themselves to a 

 gradational arrangement, which, however, is not the same as for the 

 cranial index. No. 642 possesses the least width of palate posterior to 

 the fifth tooth, the transverse diameter gradually increasing in skulls 

 646 and 643. The Nebraska skull in this character follows no. 643. 



In none of the skulls contained in the Museum of History, Science 

 and Art is the fifth superior tooth as little removed from the middle 

 of the postpalatine notch as in Brown's specimen, or as far removed 

 as in the skull described by Cockerel! 



The flattened dorsal surface of the brain-case, outlined by the 

 occipital crests, varies in width irrespective of the degree of slender- 

 ness of the cranium. In the slender skulls the width of this sur- 

 face may be equal to or greater than in some of the broader types. 

 In skull 707 the dorsal surface of the cranium is very wide, thus differ- 

 ing from the remaining skulls from the asphalt beds and resembling 

 closely Mylodon robustus as figured by Owen. 6 



* Brown, B., Bull. Amer. Mus. Nat. Hist., vol. 19, pp. 569-583, pis. 50-51, 1903. 

 s Science, n.s., vol. 39, pp. 761-763, 1914. 



Owen, B., Description of the skeleton of an extinct gigantic sloth, etc. 

 (London, 1842), pi. 3. 



