222 bulletijst 16 9, united states national museum 



All five groups are basically similar in the astragalus. The early 

 Hyopsodontidae probably are most primitive in this part, and surely 

 the characters that they share with Tetraclaenodon must be taken as 

 primitive. From this point of view, Pantolambda is much the most 

 divergent. Tetraclaenodon and the early hyopsodontids resemble 

 each other very closely, almost the only differences being the greater 

 elevation of the crests and excavation of the trochlea and probably 

 the reduction of the cuboid contact in Tetraclaenodon Both these 

 characters may be incipient speciahzations, and both are much em- 

 phasized in later phenacodonts and not in later members of other 

 groups. The longer, or relatively narrower, body in the hyopsodontids 

 is probably of slight significance. Hemithlaeus is very close both to 

 Tetraclaenodon and the hyopsodontids. Its sUghtly shorter neck, 

 almost its only pecuharity with respect to the more primitive con- 

 dylarths, can hardly be supposed to make this a "taligrade" astragalus, 

 especially as the shortness is only relative and the neck is, in fact, 

 well developed and typically constricted. The same statement applies 

 to Periptychus, the neck of the astragalus being about the same in 

 that genus and definitely more condylarthran than ''taligrade" in 

 character. All the other characters of the astragalus are condylarth- 

 ran except that the cuboid facet is about intermediate between the 

 most primitive known condylarth and amblypod conditions. 



Pantolambda has a much more primitive astragalus than Coryphodon, 

 yet the table clearly shows that it diverges farther from the primitive 

 condylarthran condition than does Periptychus. This divergence 

 consists chiefly of the appearance in rudimentary form of characters 

 greatly emphasized in Coryphodon. Despite the fact that he himself 

 abandoned it, Matthew's argument of 1897 in favor of considering 

 Periptychus as a condylarth and Pantolambda as an "amblypod" 

 seems to be as valid now as when he wrote it, indeed more so, for he 

 was not then fully aware of the distinctions between these two genera 

 now brought out. 



Periptychus does, of course, make some approach toward the so- 

 called amblypods in Hmb structure, but this is far from reaching 

 identity, and, being only vaguely or not at all seen in smaller con- 

 temporaneous allies of Periptychus, may indeed be only convergent 

 and largely conditioned by size and mode of locomotion. Conver- 

 gence is the more likely in such forms that have not in any case come 

 far from a purely primitive type of ungulate limb structure. Similarly 

 Pantolambda is much more primitive than Coryphodon or other, later 

 allies in limb structure, but it shows the beginning of the so-called 

 amblypod type, and the approach is as much toward all or any primi- 

 tive ungulates as specifically toward Periptychus and its allies. Pat- 

 terson (1934) has also pointed out that the hmb structure of Bary- 

 lambda tends to link Pantolambda with the coryphodonts. 



