lOO Bulletin American Museum of Natural History. [Vol. XIV, 



stage is reached where the two diameters are about equal, then, 

 as the antero-posterior becomes still more shortened, the trans- 

 verse exceeds it. In every series this variation in ratio seems 

 always to be most advanced in m^- and m^. This is evidently 

 due not only to the order in which the teeth of the horse come 

 into use, whereby the first to appear would at a given stage be 

 most worn, but also, as is shown by an examination of Table I 

 (p. 98), because the range of reduction of the antero-posterior 

 diameters is greater in the molar than in the premolar teeth. 

 M^ is always the most advanced, as it comes into use before any 

 of the others of the permanent set.' 



(2.) Ratio of antero-posterior diameter of the protoconc to the antero- 

 posterior diameter of the crown. 

 The antero-posterior diameter of the protocone, being, like the 

 transverse diameter of the crown, practically unchanged through 

 wear, also holds to the ever-changing antero-posterior diameter 

 of the crown in the old and much worn tooth a very different 

 relation from what it did when the tooth first came into use. 

 Thus, it may happen that in a little worn tooth the antero-posterior 

 diameter of the protocone is much less than half that of the entire 

 crown, but may h^coxix^ greater than half ^Vx^ diameter when the 

 tooth has become much worn in consequence of this shortening 

 of the antero-posterior diameter of the crown. 



2. On the molar-premolar series as a whole. 



The shortening of the antero-posterior diameters of all the other 

 teeth in the series, except p^, is not nearly compensated by the 

 lengthening of this diameter in m-^, hence it results that the series, 

 as a whole, becomes much shortened and the teeth from behind 

 crowd forward toward p^ which retains the same relative position 

 in the skull, so that m^ shows the greatest displacement and the 

 discrepancy in length is all taken from the posterior end of the 

 series. The gap which would otherwise be left in the maxillary 

 bone behind m^ becomes gradually filled in with a new growth 

 of bone, as the teeth shift forward, leaving a flattened ridge 

 which is continuous with the rugose prominence or ridge which 



' '^'h^'^^°''der^of appearance or^eruption of the permanent teeth of the large species from 

 -^.^j.v ._ .1 r^ 1 ■ for iT, cabalhis, and is: first, m-"-, second, m*, 



probably the order in all other fossil species of 



J...,, uiuci ui dp^icurance or eruption 01 tne permanent teeth ot the large : 

 u^^j^ *^' ^'^"^^^'l '^ ^^^ ^^xa& as Owen has given for E. caballus, and is : first, m-^, second, m* 

 third, p2, fourth, p3, fifth, pi, sixth, m^. This is probably the order in all other fos! "' 



thisgenus. 



