EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



609 



patella, and many of the proportions and characters 

 of its primitive subcursorial ancestors. 



The femora of the middle Eocene palaeosyopines 

 (Palaeosyops, Limnohyops) and manteoceratines 

 (Mesatirhinus, Manteoceras) are seen from Figure 516 

 to display many special titanothere characters in 

 common. 



The chief characters of the femur of the titanotheres 

 are the following, the comparisons shown being made 

 chiefly with the femur of the contemporary Amyno- 

 dontidae, which are animals of the same size. 



posterior side is recurved, surrounding a deep, sharply 

 characteristic pit. The second trochanter, t", is 

 relatively less prominent than in primitive rhinoc- 

 eroses; the third trochanter, t'", is placed high on 

 the shaft, as in all the Equidae and in the primitive 

 Rhinocerotidae (Hyrachyus) ; in Eocene titanotheres it 

 never extends down to the middle of the shaft as in 

 the progressive Rhinocerotidae; the third trochanter 

 is less extensive than in Hyracodon or Amynodon. 



4. The second and third trochanters are more 

 nearly opposite each other, as in the primitive Equi- 



FiGURE 516. — Femora and tibiae of middle Eocene titanotheres 



C, Mesatirhinus petersoni; D and E, Manteoceras manteoceras; F, Telmatherium uliimum. 

 One-sixth natural size. 



1. The femur is longer than the tibia. This differ- 

 ence is seen in the early mediportal types and increases 

 with the graviportal adaptation. 



2. The head of the femur is primitively more 

 spherical but becomes progressively flatter or less 

 spherical. 



3. The shaft is rather straight and flat and has a 

 slight forward curvature inferiorly. The great tro- 

 chanter, t' , is not very high, being but slightly raised 

 above the level of the head and relatively smaller 

 and less prominent than in Amynodon. Its inner 



dae. This is a peculiar and rather characteristic 

 feature. 



5. The external patellar ridge is smaller and less 

 produced superiorly; the distal condylar surfaces for 

 the tibia are somewhat flatter; the internal one 

 is larger. 



A most characteristic feature is the patellar trochlea. 

 In the Palaeosyopinae the knee is straighter; in the 

 Manteoceratinae it is more flexed. In the palaeosyo- 

 pine subgraviportal types (Palaeosyops, Limnohyops) 

 the patellar trochlea (fig. 518) is more vertical and 



