762 



SCIENCE 



[N. S. Vol. XXXIX. No. 1012 



In 1909 CockerelP briefly described a skull 

 from Walsenburg, Colorado. Brown believed 

 it to be distinct from Paramylodon and as- 

 signed it to the genus Mylodon. Some doubt 

 as to the validity of the genus Paramylodon 

 was subsequently expressed by Osborn,* though 

 Allen, 5 in a recent paper, accepts the deter- 

 mination of Brown. 



The diagnostic characters of the genus 

 Paramylodon were stated by Brown as follows : 



Skull elongate; muzzle inflated; dentition |; 

 first upper molar the largest of the series; last 

 lower molar trilobate; first lower molar without 

 opposing tooth. 



The nineteen skulls from Rancho La Brea 

 are similar to the Nebraska and Colorado speci- 

 mens in the elongation of the head and the 

 inilation of the muzzle. In several of the 

 California specimens the teeth have fallen 

 from the sockets, the study in such cases 

 being restricted to the alveolar outlines. The 

 series arranges itseK as follows: 



1. Four skulls with four teeth on both sides. 



2. Six skulls with four teeth on one side 

 and five teeth on the opposite side. 



3. Nine skulls with five teeth on both sides. 

 The superior dentition varies, therefore, 



from four to five teeth on each side; and this 

 variation appears to be independent of the 

 age of the individual. It follows from the 

 variable presence of the first tooth, that it is 

 the second superior tooth which is the largest 

 of the series. Upon the presence of the first 

 superior tooth depends, also, the nature of 

 occlusion with the first inferior tooth. 



The fourth inferior tooth, in a series of five 

 lower jaws, is most distinctly trilobed in an 

 individual of the first group. The tooth ap- 

 pears slightly less trilobed in individuals of 

 the second and third groups, and may be two- 

 lobed in the last group. 



Judging from the variation in the large 



3 Coekerell, T. D. A., Univ. Colo. Studies, Vol. 

 VI., No. 4, pp. 309-312, 1909. 



•* Osborn, H. F., "The Age of Mammals in 

 Europe, Asia and North America, ' ' MaemUlan Co., 

 p. 457, 1910. 



5 Allen, G. M., Mem. Mus. Comp. Zool. Uarv. 

 Coll., Vol. XL., No. 7, pp. 319-346, 1913. 



series from Rancho La Brea, the form de- 

 scribed by Brown, as well as the remains from 

 the asphalt deposits, should apparently be 

 placed in the genus Mylodon. The earlier 

 authors would probably have arrived at a 

 similar conclusion, had this extensive collec- 

 tion been available. Allen considers the Colo- 

 rado skull as belonging to Mylodon harlani 

 Owen. Possibly the specimens from Nebraska 

 and Colorado are closely related specifically to 

 the Rancho La Brea remains. At present the 

 writer will only state that the inferior dental 

 series of several mandibles from Rancho La 

 Brea bear a close resemblance to the type 

 specimen of M. harlani. 



A third type of ground-sloth from North 

 America, which has been considered with the 

 Mylodontidffi, is the genus Morotherium de- 

 scribed by Marsh" in 1874 from remains found 

 in Alameda county, California. Marsh con- 

 sidered Morotherium closely related to Mylo- 

 don and Megalonyx. It was distinguished 

 from Mylodon by the absence of the depression 

 in the head of the femur for the ligamentum 

 teres. The humerus of Morotherium differed 

 from Megalonyx in the absence of the supra- 

 condylar foramen. In 1899 Merriam'' de- 

 scribed a humerus from Pleistocene strata 

 near Tomales Bay, California. It resembled 

 closely the known portion of Marsh's speci- 

 men and was referred to Morotherium, al- 

 though the validity of the genus was ques- 

 tioned by the writer. 



A comparison of the Tomales Bay humerus 

 with several specimens from Rancho La Brea 

 indicates that the former specimen undoubt- 

 edly belongs to Mylodon. The writer has not 

 seen the type (femur) of Morotherium. As 

 stated by Marsh, it resembles Megalonyx in 

 general shape. The specimen, as figured, ap- 

 pears to be quite different in shape from the 

 Mylodon femurs of the asphalt deposits. The 

 absence of the notch for the ligamentum teres 

 can hardly be considered as generically dis- 

 tinctive of Morotherium, as it may be absent 



s Marsh, O. C, Amer. Jour. Sci., Vol. 7 (3), Art. 

 XUX., pp. 531-534, 1874. 



7 Merriam, J. C, Bull. Geol. Soc. Amer., Vol. 11, 

 pp. 612-614, 1899. 



