294 THE UNIVERSITY SCIENCE BULLETIN. 



On investigating the history of the mare I found that in the 

 nine times she had produced foals she had given birth to twins 

 (mules) on two other occasions, the fifth and ninth times (see 

 plate I). In addition, her second foal (IV3) had given birth 

 to twins (horses), and also her own half sister (IIIio) had 

 produced twins, though the latter came too soon, being **slip- 

 ped" at nine months gestation. Evidently there must have 

 been present in the family the trait of twinning handed down 

 from mother to daughter, and through three generations, 

 starting with the Oregon "herd" mare. 



That this twinning is of the dizygotic sort is proven by the 

 differing colors and markings in all of the cases, and in two 

 cases by differing sexes. In the first pair (IVe and IV7) both 

 were males; but one was black, the other brown. The black 

 one lived only two days. In the second case occurred the mule 

 and horse pair (IVio and IVn), which certainly must have 

 been dizygotic. They differed in sex and in marking, though 

 both were bay, but one a light bay (mule), the other darker 

 bay (horse). In marking, the mule had a small crescent-like 

 white blaze to the right of the center of the forehead (fig. 4). 

 The horse was of uniform bay color with black mane, tail and 

 feet. 



In the third case (IV12 and IV13) the twins were mules of 

 different sex and marked differently, the female (IV13, figs. 5 

 and 6) having a white spot in the forehead and in general 

 color darker, with less of the light tan color characteristic of 

 mules about the muzzle and under-barrel parts, while the male 

 had no white markings, was generally lighter in color, and 

 had, as the photos will show, more of the light tan color about 

 the muzzle and ventral parts. 



In the fourth case (IVu and IVir.) , in which twins were pro- 

 duced by a half sister (IIIio) of the mare III9, the foals were 

 aborted at between 8 and 9 months gestation. The sexes, 

 colors, etc., were not recorded. 



The fifth case (Vi and V2 was a pair of horse twins, both of 

 the same sex but differing in color and markings, from the 

 daughter (IV3) of III9. They did not live to maturity. One 

 was a solid bay like the mother IV3 and her sire IIIi, while the 

 other was sorrel with a "bald" white face and white feet, 

 similar to the grand dam III9 and the great grand dam (Oregon 

 mare) II4, and also to IVi (figs. 3 and 4), a full brother to the 

 dam IV3. 



