566 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 48. 



On page 743 Soergel writes that in all diluvial forms of horses 

 the relative length of the upper premolar series is the same, the 

 variations being very small and irregular. The present writer has 

 made measurements and calculations to determine what is the ratio 

 of the upper premolars to the whole premolar-molar series (regarded 

 as 100) in the horses named below. 



Ratio of upper premolars to premolar-molar series. 



Equus francisci, type 54. 8 



E. hatcheri, type 51. 3 



E. laurentius, type 54.4 



E. nioorarensis, type 54. 2 



E. scotti, 10628 Amer. Mus 55.0 



E. caoallus, 174960 Nat. Mus 53.2 



E. caoallus, 172454 Nat. Mus., Arabian 56. 6 



E. hemionus, 49493 Nat. Mus 54.0 



All of the fossil species mentioned above belong certainly to the 

 early Pleistocene, E. niobrarensis and E. hatcheri were obtained in the 

 same quarry. Nevertheless, there is a good deal of variation in the 

 ratios. E. caballus, No. 174960, is a large horse with basilar length 

 of 610 mm. and therefore of the occidental type ; the ratio here falls 

 below that of E. niobrarensis, of the early Pleistocene, in all prob- 

 ability of the first interglacial stage, and still more below the little 

 horse E. francisci. Above all stands the ratio in the last horse men- 

 tioned, the so-called Arabian. 



Soergel makes the statement (p. 743) that in the lower jaw the 

 relative length of the premolars is greater in the phylogenetically 

 older forms than in the more recent. He shows that in the case of 

 E. stenonis of the late Pliocene the ratio of the premolar series to 

 the whole tooth-line is 56.0; in E. silssenbornensis, of the first inter- 

 glacial stage, 53.1 ; in E. mosbachensis, also of the first interglacial, 

 52.6 to 51.7; E. germanicusf from Taubach of the third interglacial, 

 52.1 ; and E. germanicus from the loess, 48.6 to 50.5. 



From the specimens from which the present writer obtained the 

 ratios for the upper teeth the following ratios have been determined 

 for the lower cheek-teeth, E. francisci being necessarily omitted : 



Equus hatcheri 52. 1 



E. laurentius 50. 



E. nioorarensis 53.0 



E. scotti 52.2 



E. caoallus 51. 9 



E. caoallus 53. 



E. hemionus 51. 3 



There appear in the above table no evidences that the lower pre- 

 molar series has become shorter in the horses since the beginning of 

 the Pleistocene. Furthermore, if we go back to Mesohippus bairdi, 



