346 S. CALVIN AFTONIAN MAMMALIAN FAUNA 



An imperfect second upper premolar, with, the thin, anterior edge 

 broken away and having a transverse diameter of 29 millimeters, belongs 

 in this group of large teeth. ISTearlv one-third of the crown has been 

 worn off by use ; the part remaining measures 65 millimeters in length. 



Among the recognized species of Pleistocene horses the teeth of the 

 Gladwin horse and the others above noted agree best in size with Equus 

 scotti Gidley, from the Sheridan beds, Rock Creek, Brisco county, 

 Texas, 6 though tooth number 117 is practically identical in size and other 

 details with the superior third and fourth premolars referred by Gidley to 

 Equus pacificus Leidy. 7 Comparing the Gladwin teeth with the measure- 

 ments given for Equus scotti in the American Museum Bulletin, volume 

 xiv, page 136, the close agreement becomes apparent: 



p 2 . p 3 . p*. ni 1 . m. in 3 . 



Antero-posterior diameter : 



Equus scotti 43.0 34 33 30 31 31 



Gladwin horse 43.5 33 31 25 26 32 



Transverse diameter : 



Equus scotti 30.5 33.0 33.0 30 29.0 24 



Gladwin horse 28.7 32.0 32.5 30 28.5 26 



The differences that appear in making these comparisons may be ac- 

 counted for on the basis (1) of individual variations and (2) of differ- 

 ences in the amount of wear which the teeth of the two animals has 

 undergone. The measurements of Equus scotti given on page 136 of the 

 work cited are those of an individual "in which all the teeth have come 

 into full use/ 7 but presumably an animal comparatively young. The 

 teeth of the Gladwin horse, on the other hand, have been worn down to 

 about half of their original length. The greatest discrepancy appears in 

 the antero-posterior diameters of m 1 and m 2 .and in the transverse diam- 

 eters of p 2 and m 3 . Applying Gidlej^s "Laws governing the changes of 

 diameters of the tooth crowns/ 7 formulated on page 99 of the American 

 Museum Bulletin already quoted, the differences are largely, if not 

 wholly, explained. After the molar-premolar series comes into full use, 

 according to law (1), the antero-posterior diameter of each of the inter- 

 mediate teeth diminishes at first very rapidly, and then more gradually 

 to the roots. Differences even as great as those seen in m 1 and m 2 are to 

 be expected. The antero-posterior diameter of m 3 in the Gladwin horse 

 accords with law (3). The differences in transverse diameters of the 

 first and last teeth of the series exemplifies that part of law (4) which is 

 expressed in the clause "p 2 gradually diminishes, while m 3 increases in 



8 Bulletin of the American Museum of Natural History, vol. xiv, p. 134. 

 7 Op. cit, p. 117. 



