THEORIES AS TO ORIGIN, ANCESTRY, AND ADAPTIVE RADIATION 



767 



exposed. It is only in upper Eocene perissodactyl 

 skulls that we have found an osseous and tubular 

 tympanic bone, as noted above. 



The brain case of the cranium as seen from above 

 (fig. 695, A, C, D) does not appear to be much larger 

 relatively in the primitive horses and tapirs (B, C) 



than in the condylarths (A); the sharp constriction of 

 the brain anteriorly seems to be as marked in Systemo- 

 don as in PTienacodus. The actual brain structure can 

 not be known until a natural cast can be obtained. 



A summary of the skull characters as observed in 

 the various types is as follows: 



Thirty primitive cranial characters possessed in common ly two species oj Eocene Condylarthra and ly fourteen 



species oJ Perissodactyla 



From the above general descriptions and table 

 it is clear that the prevailing or primary type of 

 perissodactyl skull is that approached by the 

 horse (Eohippus), the tapirs {Systemodon, Isectolo- 

 phus), the titanotheres {Lambdotherium, Eotitanops). 

 In this primary type we observe that the face. 



including the orbits, is always longer than the cra- 

 nium. We may regard this as a primitive char- 

 acter. In the middle Eocene Lophiodon and all 

 the middle Eocene titanotheres the cranium is longer 

 than the face, a character which thus appears to 

 be secondary. 



