FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 223 



The later history of the astragalus and feet in general in these 

 groups is not entirely pertinent but may be mentioned. In the 

 phenacodonts, culminating in Phenacodus itself, the limbs became 

 considerably speciahzed in an inadaptive cursorial du-ection. Side 

 toes were moderately reduced, the limbs became or remained moder- 

 ately slender, and the feet digitigrade, carpus and tarsus serial. The 

 astragalus differs markedly even from that of the closely allied but 

 earher Tetraclaenodon. The trochlea becomes very long, the foramen 

 and emargination are lost, the crests both become high and sharp, and 

 the head becomes more spherical and loses contact with the cuboid. ^^ 

 The hyopsodonts were remarkably conservative. As far as we know 

 them, the limbs of Hyopsodus differed extremely little from those of 

 its long antecedent Middle Paleocene relatives. An astragalus of 

 Hyopsodus from the Eocene is almost identical with that of a Paleocene 

 hyopsodontid here described except for the quite unimportant details 

 of having the body somewhat less elongate and the head shghtly more 

 spherical. The Anisonchinae and Periptycliinae have no known 

 descendents after this stage (except for a few scraps in the early 

 Upper Paleocene apparently not generically different from those of 

 the Middle Paleocene). The amblypod astragalus became very 

 markedly modified in Coryphodon. Its limbs are highly graviportal 

 throughout, and the astragalus is profoundly modified and convergent 

 toward some other graviportal types. 



The present conception of this order is as follows: ®^ 



Order Condtlarthra: 



Family Hyopsodontidae: 



Subfamily Mioclaeninae 1,t , , • - .i • ■^■ 



„ , , .,' TT 1 X- I Members of a persistently very primitive 



Subfamily Hyopsodontinae J 



group. Small, possibly insectivorous (in habits, not affinities) ani- 

 mals with simple, low-crowned, bunodont teeth and clawlike unguals. 

 This longest-lived group is also in almost all respects the least 

 specialized. Lower Paleocene to upper Eocene. North America. 

 Family Phenacodontidae: A progressively more cursorial and probably 

 more strictly herbivorovis group, generally analogous within this much 

 more primitive and nonadaptive radiation to the early progressive ungu- 

 lates (especially perissodactyls) that replace them in the Eocene. Teeth 

 brachyodont, becoming polybunous with some slight tendency toward 

 lophiodonty. Lateral toes becoming somewhat reduced and unguals 

 flattened into hoofs. Middle Paleocene to lower Eocene. North America, 

 South America, Europe. 



" Some of these characters were supposed to be typical of the Condylarthta, because Phenacodus was 

 ihe only adequately known genus when the order (or suborder, then) was first defined, and it may seem 

 strange to consider Phenacodus as a peculiar and in many respects atypical condylarth. It is, in some 

 sense of the word, technically a "type" of the Condylart.hra, but it is definitely n(n typical throughout. 

 Taxonomy has many such cases in which a natural group was recognized and named, even though in the 

 first instance it was largely based on a form later found to be marginal in it. 



»3 Matthew has given excellent diagnoses of all the groups concerned. These characteristizations are 

 meant to be explanatory, not formally diagnostic. 



