310 The American Naturalist. [April, 



developed protoloph. In Mixtotherium the hypocone is selenoid in 

 structure as in Dacrytherium, but this cusp is much smaller and it is 

 much less extended internally than in that genus. I emphasize especi- 

 ally the large development of the mesostyle, and the presence of a pro- 

 toloph, characters of the upper molars of Mixtotherium which differs 

 decidedly from those of Dacrytherium. The structure of the fourth 

 upper premolar in MhMherium resembles somewhat that of Agrio- 

 chcerm, but differs from this genus in the presence of the postero- 

 internal cusp. In Dichodon Owen, the complication of the fourth 

 upper premolar is carried still further than in Mixtotherium, as in 

 Dichodon this tooth is completely molariform and selenodontin struct- 

 ure. However, I believe, that Mixtotherium has no close affinity with 

 Dichodon, as the structure of the skull and dentition in Diekodon is 

 quite modernized. 



The lower jaw in Mixtotherium is rather short and deep below the 

 last lower molar, these characters differ strikingly from those of the 

 Anopktheriida, where the jaw is very slender and elongated. The 

 mandibuke are strongly ankylosed at the symphysis as in the primitive 

 pigs, Acotherulum and Cebochcerus, this is a character I believe seldom 

 found in the Mammalia outside of the Primates. The last lower pre- 

 molar in Mixtotherium is intermediate in structure between a last milk 

 tooth and permanent molar. It consists of an antero-median cusp, 

 bunoid in form, and posterior to it, of two external crescents and two 

 flattened internal elements. The structure of the inferior true molars 

 is like that of Dacrytherium. 



It appears to me that the genus Mixtotherium is of importance phy- 

 logenetically, and demonstrates how closely the Suillines and Anoplo- 

 theroids are related. In the characters of the skull and the large 

 development of the canines Mixtotherium is mere like the pigs, but show- 

 ing affinities to the Anoplotheroids in the form of the brain case. The 

 structure of the molars, as already shown, resemble very closely those 

 of the Anoplotheriidoz and have gone one step further in their specializa- 

 tion by the development of a well defined protoloph. 



Schlosser in his paper, " Stammesgeschichte der Hufthiere" speaking 

 of the origin of the Sullines remarks " die Herkunft diese Stammes ist 

 noch in vollstandiges Dunkel gehiillt, nursoviel diirfen wir alssicher 

 annehmen, dass derselbe wohl von der gleichen Grundform ausgegan- 

 gen ist wie der der Suiden." The Oreodonts are considered by Scott to 

 be related to the Anoplotheroids, and if this be the case it is not strange 

 that the skull of Mivtotherium resembles that of Oreodon. The genus 

 Protoreodon of the Uinta or Upper Eocene, has the five lobed superior 

 molar typical of the Anoplotheroids, and the primitive Suillines. 



