[go I.] Gidley, The North' America7i Species of Eqmis. 



lOI 



marks the posterior extension of the maxillary bone beyond m-^; 

 hence the length of this posterior extension of the maxillary 

 depends principally upon the age of the horse. In passing from 

 the young to the old stage, there is also a marked change in the 

 relative position, with respect to the molar teeth, of the anterior 

 projection of the maxillary ridge, the post-palatal foramina, and 

 the anterior projection of the post-palatal notch, — all appearing 

 relatively more posterior in the old individual.' 

 B. — Individual Variability. 

 a. Degree of complexity of the enamel foldings of the teeth. — The 

 examination of a large number of specimens of E. caballus, many 

 of them of about the same age, shows a great variability in this 

 character aside from that caused by wear; this indicates that in- 

 dividual variability also has a very wide range, and no two speci- 

 mens can be found, even of the same age and species, in which 

 the enamel foldings of the corresponding teeth are exactly alike, 

 and even corresponding teeth of the opposite sides of the same 

 skull often show slight differences in the number and style of the 



Fig. 4. Superior molars m^, E. caballus. A and B of different 

 individuals of about the same age, showing variation in plication. 



minor enamel folds. Fig. 4 (.4 and i>) represents the corresjjond- 

 ing teeth (m-*-) of two very large draught horses of the same variety, 

 with apparently only about a year's difference in age, the one 

 represented by A being the younger. These teeth show consider- 

 able difference in the degree of complexity of the enamel folding, 

 which may be due in part to their difference in age, but specimens 

 can be found in individuals of the same age in which the differ- 

 ence is just as great. The corresponding tooth of the Texas 

 pony (PI. XIX, Fig. D) is much simpler than either of the above, 

 yet the degree of wear of the teeth shows that this tooth was 

 apparently intermediate in age between those of A and B, Fig. 4. 



