96 



BULLETIN 169, UNITED STATES NATIONAL MUSEUM 



that is, not significantly different from P. montanus or mediaevus but 

 much higher than in any other known species of this or closely related 

 genera. Serrations P4, type, 14 (two other specimens 14 and 15). 

 P4 very short and high in lateral contour, rising well above the grinding 

 plane of Mj. 



Remarks. — Except for its somewhat more elevated P4, this closely 

 resembles P. montanus in its Icnown proportions and other morpho- 

 logical features, but it is much smaller (deviation of length P4 from 

 mean of P. montanus —2.0, nearly five times standard deviation of the 

 latter), too much so to consider it a small variant of montanus, despite 

 the considerable variabilitv of the latter. 



Figure 9. — Ptilodus, comparative outline drawings of lower dentition: a, P. douglassi Simpson, U.S.N.M. 

 no. 9795; b, P. gidleyi Simpson, U.S.N.M. no. 9763; c, P. sinctairi Simpson, U.S.N.M. no. 9770. Crown 

 and external views, all drawn as if left lower jaws (a and c reversed from right lower jaw). Three times 

 natural size. 



P4 of this species is of the same size as in P. trovessartianus. Ma- 

 terial is insufficient for full analysis, but if we assume the variability 

 of both not to be much greater than in P. montanus, the shorter Mi 

 of ?P. gidleyi*^ and the resulting larger ratio length P4: length Mi 

 are surely significant. These same differences distinguish it even 

 more sharply from the somewhat larger species ?P. douglassi. No 

 others resemble it closely. 



Table 15. — Measurements of individual specimens of ?Pti]odus gidleyi 



«' This is not exactly determinable from the specimen, but the error of measurement can hardly exceed 

 0.2 mm and is almost surely less. Alveoli in a referred specimen also indicate a very short Mi. 



