BIG BADLANDS OF SOUTH DAKOTA. 



75 



third molar is just appearing above the gum. In Fig. 5 a very much 

 older individual (No. 10144, Princeton University Geological Mu- 

 seum) is seen, in which the open valley is retained in both the third 

 and fourth premolars, both of which are well worn, as are also the 

 anterior molars. The third molar is fully erupted and partly worn. 

 Both these specimens are fractured in such a way as to show the 

 absence of germs of replacement teeth above the posterior premolars 

 and I can not escape the conclusion that we are here dealing with per- 



FiG. 5. Hyracodon leidyaniis, No. 10144 Princeton University Geological 

 Museum. Upper premolar-molar series from p ^-m ^ of the right side, crown 

 view, three fourths the natural size. 



manent teeth. In Hyracodon the milk dentition is more advanced in 

 crown-pattern development than are the permanent premolars, even 

 milk p- having an unblocked transverse valley (Figs. 6, 7). These 

 molariform milk teeth have lower crowns and thinner enamel than 

 their successors, and in p- there is a prominent style from the cingu- 

 lum at the entrance of the transverse valley. Mr. Gidley has called 

 my attention to certain specimens in his charge in which no germ 

 teeth of the replacement series appear beneath the milk premolars 

 and suggests that they " were very late in forming, but when started 

 grew very rapidly to replace the milk dentition which seems some- 

 times to have persisted until all the true molars have come into use." ^'^ 



Fig. 6. Hyracodon sp.. No. 12679 Princeton University Geological Mu- 

 seum. Milk dentition and m ^ and - of the left side, crown view, three 

 fourths the natural size. M ^ had germinated but is not preserved with the 

 other teeth. Germs of the replacement series appear in the maxillary above 

 the first and third milk premolars. 



^^ Personal letter. 



