No. 545] 



ORIGIN OF UNIT CHARACTERS 



275 



subsidiary folds in the grinders of the mule are simpler 

 than those in either the horse or the ass. The grinder 

 of the mule would be pronounced by any systematist not 

 knowing its mixed parentage to belong to the ass rather 

 than to the horse, especially in the absence of the "pli 

 caballin" (fold 5), in the form of the hypo style (hs, fold 

 6), in the smaller size of the protocone (pr), the large 

 size of which is very distinctive of the horse. A very 

 detailed study and comparison of the grinding teeth in 

 the horse, ass and mule made by an independent ob- 

 server, Dr. W. K. Gregory, gives the following result: 



It would be especially desirable to compare the -ante 

 enamel characters in the hinny, which is a cross be- 

 tween the male horse and the female ass, in which it is 

 well known that the E. caballus and E. asinus char- 

 acters are differently distributed. 



Summary.— Out of the 28 characters examined in the 

 skull and teeth of the mule, 18 are distinctly derived 

 either from one parent or the other with very slight, if 

 any, tendency to blend, 10 characters show a distinct 

 tendency to blend. 



