308 ON THE EXTINCT MAMMALIA OF 



their relative position, angularly convex and sloping externally, concavely excavated 

 internally, and are acutely crescentoid at the summit. Of the inner secondary lobes, 

 the anterior is much the larger, and is pyramidal in form, with a twin pointed summit. 

 It springs from the crown at the conjunction of the principal lobes, and is continuous 

 with their contiguous horns. The posterior of the secondary lobes is conical, and 

 siDrings from the crown in conjunction with the back horn of the posterior principal 

 lobe. The front horn of the anterior principal lobe curves inward, downward and 

 backward to the base internally of the anterior secondary lobe. 



A basal ridge, nearly continuous, bounds the crowns of the lower molars 

 externally, in front, and behind. In the latter position it rises inwardly, and termi- 

 nates in a tubercle spi'inging from the conjunction of the posterior principal and 

 secondary lobes. 



In comparing the lower molars with those of the Horse, it is evident that the 

 principal lobes of the crown correspond with the external crescentoid columns in the 

 latter, the anterior secondary lobe with the anterior internal twin column in the 

 Horse, and the posterior secondary lobe, with its contiguous tubercle terminating the 

 basal ridge, corresponds with the posterior twin column in the teeth of the Horse. 



The hinder lobe of the last molar tooth I'esembles the principal lobe in advance, 

 reduced in size. The anterior large molar is smaller than the others, and is narrowed 

 at its fore part. 



As the lower molars are worn, crescentic tracts of dentine appear at the summits 

 of the principal lobes, and minute islets on those of the secondary lobes. The denti- 

 nal tracts gradually widen and become continuous throughout on the summits of all 

 the lobes. When the teeth are about one-third worn away, the enamel lines on the 

 masticating surface pursue a general course approaching in appearance the character- 

 istic arrangement observed on the worn teeth of the Horse. 



The temporary dentition of AncMtherium is most probably like that of the Horse. 

 Specimens of jaws of A. Bairdi exhibit three temporary molars in both jaws, corres- 

 ponding with the anterior three of the permanent series, which they exactly resemble 

 in form. The first one above and below has a greater breadth in proportion with 

 the width than in their permanent successors, and their fore part is better developed. 



Since writing the above, I have had the opportunity of examining several speci- 

 mens of mutilated skulls of Anchitherium Bairdi, obtained by Dr. Hayden in his last 

 trip to Dakota, which retain the anterior extremity of the face in a state of preserva^ 

 tion sufficiently well to indicate its conformation to be the same as in the Horse. 



The premaxillaries extend upward and backward to articulate by their upper end 

 with the nasals. The ends of the latter project as in the Horse, but retain a propor- 

 tionately greater breadth and terminate in a more obtuse manner. 



