330 University of California Publications in Geology [Vol. 8 



varies in distinctness. In no. 21156, belonging to group three, this 

 lobe is as distinct as in Brown's specimen. In a younger individual 

 (no. 21165) of the same group the third lobe is but faintly developed. 

 The middle lobe of the fourth tooth appears to vary, however, in 

 individuals of group three irrespective of age. In an adult (no. 21576) 

 of group one the tendency toward development of a middle lobe is 

 much more in evidence than in no. 21165, though the lobe is not 

 nearly so prominent as in no. 21156. In the type specimen of M. 

 harlani there is a suggestion of a third lobe, as already pointed out 

 by Cockerell. 8 



SPECIFIC COMPARISON WITH OTHER MYLODONT FORMS 



It is not proposed here to enter into a detailed comparison of 

 all skull material of the Mylodontidae from North and South America. 

 Determination of the ground-sloths has often been based upon parts 

 of the skeleton other than the skull, and a discussion of the associated 

 fragments of the skull and teeth involves a study of the entire 

 skeletal structure. A full discussion of relationships of these forms 

 is reserved for the final report. 



The genotype, Mylodon harlani, is a fragmentary right ramus with 

 the last three teeth intact. The specimen was obtained from Big 

 Bone Lick, Kentucky, and determined by Owen in 1840. Figures 

 of the type as given by Leidy 10 were used in a comparison with the 

 Rancho La Brea material. 



In size and form the teeth of the majority of the Rancho La Brea 

 specimens agree closely with Mylodon harlani. The outer anterior 

 face of the third tooth in lower jaws from the asphalt beds is usually 

 slightly grooved. In the type this surface is represented as convex. 

 The fourth tooth in no. 21576 strongly resembles the corresponding 

 tooth in the type specimen. 



In 1895 Cope 11 described a superior series of teeth and an inferior 

 tooth which he referred to Mylodon harlani. In the third superior 



s Cockerell, T. D. A., A Fossil Ground-Sloth in Colorado, Univ. Colo. Studies, 

 vol. 6, p. 311, 1909. 



a Owen, R., The Zoology of the Voyage of H. M. S. Beagle, etc., Part 1, 

 Fossil Mammalia, p. 68, pi. 17, figs. 3 and 4, 1840. 



io Leidy, J., A Memoir on the Extinct Sloth Tribe of North America, Smith- 

 son. Contrib. Knowl., vol. 7, pi. 14, figs. 1 and 2, pi. 16, fig. 19, 1855. 



n Cope, E. D., On Some Plistocene Mammalia from Petite Anse, La., Proc. 

 Amer. Philos. Soc, vol. 34, pp. 458-468, pis. 10-12, 1895. 



