316 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Evolution of proportions of Palaeosyops 



[Measurements in millimeters. Level, if known, given after specimen number] 



'Specific reference doubtful. 



The accompanying table brings out the following 

 facts: 



1. The law of regular progressive increase in size as we pass 

 from P. paludosus (of Bridger B 1) through P. major (of levels 

 B 2-3), P. leidyi (C 2 to C 4?), to P. robustus. 



2. The exceptionally large P. major skull, Am. Mus. 13116, 

 from B 2 interrupts this regular increase, but its tooth row is 

 not larger than in other P. major skulls (that is, the skull and 

 the teeth are differential) . 



3. So far as the measurements are evidence, the larger speci- 

 mens of P. major overlap the smaller specimens of P. leidyi, 

 and the larger P. leidyi overlap the smaller P. robustus. 



4. P. grangeri, although it is not so advanced in its stage of 

 premolar evolution as P. leidyi and although it comes from a 

 lower level, yet exceeds P. leidyi in size. 



It 



5. P. leidyi seems to be very closely allied to P. robustus. 

 exhibits considerable fluctuation in size. 



6. P. copei, although more advanced in premolar evolution 

 than P. robustus, has a somewhat smaller dentition (differential 

 evolution) . 



7. While the premolar series remains relatively stationary in 

 length the molar series increases from 91 millimeters in P. 

 paludosus to about 102 in P. robustus (differential evolution). 



8. The fourth premolar and first molar exhibit a marked 

 increase in size, especially in width (differential evolution). 



Other conclusions from the measurements are con- 

 sidered under the heading "Differential allometrons," 

 on page 825. 



