Mylodon harlani, from Rock Creek, Texas. 377 



navicular, no apparent line of demarcation between the two 

 being visible. The navicular facet is remarkable, especially in 

 the peculiar pit-like depression in its flattened upper portion. 

 Owen does not remark this depression in his description nor 

 does be figure it, but the area is flattened. Barnum Brown 

 (p. 581) in his description of Paramylodon, however, describes 

 an astragalus precisely like the Texan 10266 which is the sub- 

 ject of this sketch, hence this seemingly variable feature 

 within the Texas group is considered by Brown to be a char- 

 acter of generic importance. 



Dimensions. 



Extreme height .- 



Antero-posterior diameter 



Transverse diameter 



Ant.-post. diameter of ascend- 

 ing process . ' 4 1 # 7 f 



10264 



10265 



10266 



mm 



mm 



mm 



111* 



110 



119'5 



129 



127-5 



131 



101* 



100 



103 



1 41-7 + 



39-5 



54 



Eatio 

 between 

 latter two 



108-6 

 103- 



103 



111-7 



Estimated. 



f Imperfect. 



The ratios between the bones bring out the relatively greater 

 depth of 10266, but especially the strength of the ascending 

 process. May not the tendency toward a ball and socket arti- 

 culation between astragalus and navicular in this specimen be 

 simply a further mechanical reinforcement of a more powerful 

 joint and therefore merely a matter of individual variation ? 



Naviculars. — Two naviculars, both from the left side, are 

 preserved, fortunately representing the two types, one with a 

 distinct rounded boss on the astragalar surface which fits into 

 the above mentioned depression, in other words Paramylodon- 

 like, and the other with the surface plane or slightly concave 

 like that of Mylodon. 



The general form of the navicular is irregularly quadrilateral, 

 with somewhat rounded angles, and is a comparatively thin ex- 

 panse of bone concave proximally, and convex distally. The 

 entire proximal surface is occupied by the astragalar facet, con- 

 cave throughout in JS r o. 10265 and convex over half its area in 

 No. 10266. Distally a large ovoid surface, evenly convex, 

 characterizes both bones. This area, which includes about 

 half the entire surface, is the external cuneiform facet and 

 adjacent to it is the small synovial surface which came in con- 

 tact with the internal cuneiform. The distal face of the navi- 



