I02 Bulletin American Museum of Natural History. [Vol. XIV, 



Although domestication may have made the range of individual 

 variability greater in E. caballus, yet a careful examination of a 

 large number of fossil teeth indicates that in the extinct species 

 as well the range for this character was great. 



b. Effect of individual variability on dimensions of the teeth. — It 

 has been shown under the topic of age variations that the trans- 

 verse diameters of all the superior molars and premolars, except 

 p^ and m^, the antero-posterior diameters of the protocones of 

 all the teeth and the antero-posterior diameter of p-2- are measure- 

 ments which change but slightly for much the greater length of 

 the crown; hence, unless specimens of the same age are taken 

 for comparison, it is in these measurements that one should look 

 for evidences of individual variability. Careful measurements of 

 the teeth of more than ten specimens of E. caballics have led to 

 the following conclusions : (i) The transverse diameters of the 

 corresponding teeth for p^ to m^ inclusive are remarkably con- 

 stant, especially in skulls of nearly the same size; the greatest 

 difference in a certain series of four skulls of large draught horses 

 examined not exceeding 2.5 mm., and in another series of three 

 skulls belonging to animals about the size of carriage horses being 

 less than i mm. (See table of measurements, p. 98.) 



The transverse diameters of m^ of the large series vary only 

 .8 mm., while in the small series all the transverse measurements 

 for this tooth are the same. The greatest difference in the trans- 

 verse diameter of m-, including both series of skulls, is only 

 1.5 mm., and adding a skull of the Texas pony to the list the 

 extreme difference between the transverse diameter of m^ of this 

 whole lot of skulls, ranging in size from the large draught horse 

 to the small Texas pony, is only 2.5 mm. It will be seen by 

 reference to the table of measurements (p. 98) that the average 

 variation of the corresponding transverse diameter for all these 

 teeth is very small considering the great difference in size of the 

 animals represented. 



It seems reasonable that much greater variations of the com- 

 paratively constant characters of the teeth would be found in E. 

 cabalhts than in the extinct species, since in this species domestica- 

 tion and breeding have caused such a very wide range in size and 

 proportions of the individuals; hence, when, in two lots of fossil 

 horse teeth, the difference between the transverse diameters of 



