Mylodon harlani, from Rock Creek, Texas. 341 



Through the courtesy of Professor J. C. Merriam of the 

 University of California, I have been able to secure by ex- 

 change a line skull of Mylodon from the Rancho La Brea 

 asphalt deposits, with which to make a comparison. This 

 skull, Cat. No. 10205, Yale Museum collection, while very 

 complete, is that of an individual somewhat smaller than any 

 under consideration except No. 10207. The asphalt specimen, 

 except for its color, has all of the aspect of recent bone, in that 

 the finest details of surface sculpture and relief are preserved. 

 The Texas specimens, on the other hand, are partially mineral- 

 ized and are somewhat abraded so as to lack this surhcial 

 detail. 



Occiput. — The occipitals of the Texas form, in keeping with 

 their greater size, are of proportionately heavier bone than 

 those in the Rancho La Brea specimen. As preserved, the 

 general outline of the bone is somewhat pentagonal, with a 

 sharply incised lower border. The median occipital crest in 

 the California specimen is here represented by a low rounded 

 ridge in No. 10261, while in 10265 it is somewhat more 

 prominent. The reduction of the crest in the former speci- 

 men may in part be due to abrasion, but its condition is mid- 

 way between that of the California type and M. robustus 

 as figured by Owen. The Brea specimen also shows on each 

 side the narrow and sharper ridge receding from the upper 

 crest as it curves downward toward the condyle mentioned by 

 Owen* as true of Mylodon robustus. These crests are but 

 faintly indicated in either Texan occiput. The shape of the 

 foramen magnum differs materially in the California and Texas 

 specimens, especially in its upper margin. In the latter there 

 is a deep median notch on either side of which the margin 

 rounds in a gentle curve to the occipital condyle. In the Brea 

 skull the notch is rounded and is bounded on either side by a 

 distinct lobe of bone, the outer margin of which rises to its 

 junction with the condyle. In this regard the Texas form 

 resembles M. robustus more nearly, as it also does in the 

 reduction of the crest. The inferior margin of the foramen 

 magnum is somewhat more acutely angled in the Texas speci- 

 mens than in either the California type or in M. robustus, and 

 the general proportions of the foramen differ in the two 

 former types, the height of the aperture in the Texas indivi- 

 dual, No. 10264, being 41'5 mm , and the breadth, 41 mm ; while 

 in the Rancho La Brea specimen the height is 32*5 mm , and 

 the breadth is 48 , 7 mm , the ratio of width to height being 

 markedly greater. 



* Owen, R., Description of the skeleton of an extinct gigantic sloth, 

 Mylodon robustus, Owen, p. 28, 1842. 



