Farr.] ^^" [May 15, 



The fauna of the Protoceras beds is unique in many ways, especially 

 in the number of new and bizarre forms that come in, some evidently 

 by migration, while others are the direct descendants of the species of 

 the underlying Oreodon beds. 



These strata are interesting, as they form a transition to the later John 

 Day beds, their fauna being intermediate between the latter and that of 

 the Oreodon beds. 



A new species of horse has been found in this formation which helps 

 very greatly in explaining the individual variations of M. bairdi, as 

 many of these are seen to be attempts in the direction of M. intermedium, 

 which is undoubtedly the direct descendant of the former. Besides 

 these two species which are seen to stand in the direct relation of ances- 

 tor and descendant there is another species, M. copei, which occurs first 

 in the strata of the Oreodon beds and is represented in the Protoceras 

 beds by larger individuals. 



Geological succession of the species : 

 Protoceras beds : M. bairdi, M. copei, M. intermedius. 

 Oreodon beds : M. bairdi, M. copei. 

 Titanotherium beds : M. bairdi. 



The genus Mesohippus occurs then in all the different horizons of the 

 "White River beds. In the Titanotherium beds it is usually represented 

 only by fragmentary remains, which, however, are unmistakably those 

 of 31. bairdi. 



The Oreodon beds have yielded most of the best material. Through 

 the whole extent of the fossiliferous strata of these beds, a vertical thick- 

 ness of at least one hundred and eighty feet, remains of M. bairdi are 

 fairly abundant. However, the remains are not well preserved, groups 

 of teeth and the larger limb bones are common, while well-preserved 

 portions of the skeleton are rare — a perfect skull has never yet been 

 found. Beside M. bairdi we get in the upper Oreodon beds a new spe 

 cies which has been described as 31. copei.* The Protoceras beds have 

 yielded only fragmentary remams of 3f. bairdi. This species does not 

 represent the main line of descent during this epoch, but it is here taken 

 up by 3f. intermedius while the former still persists as a side line. We 

 also get 31. copei, which continues on from the Oreodon beds and is now 

 represented by larger individuals. 



Of 31. bairdi newr}y the entire skeleton is represented by material in 

 the Princeton collection. 



The skull has been quite fully described by Leidy,f and the skeleton 

 has been the subject of an exhaustive paper by Prof. Scott, | but when 

 this paper was written the entire skeleton had not yet been found and 

 the incisor teeth of upper series are the result of explorations of the 

 summers of 1894 and 1895, so some points in the description will be new, 



*Osborn ai)cl Wortmai), /'«?/. Aw. Miis., Vol. vii, pp. 35G-3.5''. 



t Tlie Extinrt Mammalian Funna of Dakota and Nebraska, Philadelphia, 1809. 



XJourii. of Morvhvloyy, Vol. v, No. 3, December, 1891. 



