EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



611 



This law of the dominance of teleogenetic (adaptive) 

 over syngenetic (ancestral) character is in force in 

 every single element of the pes as in the manus. 



For example, the articulations between the astra- 

 galus and calcaneum exhibit both a mediportal type 

 resembling that of tapirs, of light-limbed rhinoceroses, 

 and of paleotheres and a graviportal type resembling 

 that of other graviportal perissodactyls. We are 

 therefore again compelled to examine minor and less 



cuboid, respectively. In one specimen of Palaeosyops 

 Mts IV abuts against the ectocuneiform, but this is 

 unusual. Similarly Mts III occasionally doe.-i not 

 abut against the cuboid. 



In the two middle Eocene groups, namely, the 

 Palaeosyopinae and the Manteoceras-Dolicliorhinus 

 group, it is possible by very careful study to discover 

 distinctive generic characters by which we may sepa- 

 rate every bone of the manus and of the pes, although 



Figure 518.- 



Angulation of the knee joint: relation of patellar facet to long axis of femur 



Amynodon; Cae, 



A, C, D, and E, one-fourth natural size; B, one-half natural size. Lm, LimnoJiyops; Hy, Hyrachyus; Ms, MesatiThinus; A 



Caenopus. 



conspicuous characters in order to discover the real 

 syngenetic family resemblance to be found in each 

 element.^" 



Among family characters of the pes of the titano- 

 theres are the following: 



1. The small fibulocalcaneal facet, as in the Equidae, 

 the fibula barely passing upon the calcaneum in the 

 extreme extension of the foot. A median pit on the 

 astragalus checks the flexion of the tibia by receiving 

 its posterior process. 



2. The entocuneiform is very large, articulating on 

 the inner side of the mesocuneiform, of Mts II, and 

 of the navicular, and freely projecting backward from 

 the pes like a pisiform. 



3. The mesocuneiform is invariably a very small 

 bone as in primitive mammals. 



4. The ectocuneiform is a large element. 



5. Mts II and III typically abut on the outer 

 proximal facets against the ectocuneiform and the 



'• The significance of the astragalocalcaneal facets as family characters in Perisso- 

 dactyla was first pointed out by Osborn in the article "Evolution of the ungulate 

 foot" (Scott and Osborn, 1890.1, pp. 531-569). 



through convergence and inheritance the bones are 

 often brought to resemble each other closely. The 



h<^ 



Figure 519. — Inner side view 



of left fibula 

 A, Palaeosyops sp.; B, Limnohyops 



monoconusf ; C, Brontotherium leidyl. 



One-sixth natural size. 



chief subfamily distinctions in typical genera are 

 stated on the following page. 



