534 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 48. 



lake of E. hatcheri, there is in the molars, but more conspicuously in 

 the premolars, a reentering double fold, resembling somewhat an M. 

 The homologue of this is usually present in the domestic horses, but 

 is rarely so broad, and it is often only a simple fold. 



Wilckens * concluded that the complication of the enamel surround- 

 ing the lakes of the upper cheek-teeth is far simpler in the Arabian 

 horse than in what he called the occidental horses. His figures 

 (pi. 9, fig. 5) represents the lakes as surrounded by enamel with 

 little crinkling in the adjacent borders. In the Davenport Arabian 

 horse the enamel of the lakes is more complicated than it is in many 

 of the large domestic horses. 



The anterior inner valley of the upper cheek-teeth of E. hatcheri 

 is directed less outward than in the corresponding teeth of E. cabal- 

 lus; and, especially in the premolars, does not reach so near the 

 center of the tooth as it does in the domestic horses. 



The reentering fold in the head of the anterior inner valley, so 

 commonly seen and so deep in the premolars of the large domestic 

 horses, is shallow in the premolars of E. hatcheri and absent in the 

 molars, except in the last one, where it is a mere nick. 



In the lower cheek-teeth of E. hatcheri the valley (pi. 34, fig. 2) 

 seen at the middle of the outer face is shallower than in the greater 

 number of domestic horses. In the premolars of the latter it reaches 

 usually more than half way to the enamel of the inner face, while 

 in the premolars of E. hatcheri it does not penetrate the tooth so 

 deeply. In the molars of most specimens of E. caballus this valley 

 forces itself between the longitudinal expansions of the two inner 

 valleys and almost reaches the enamel of the inner face. In E. 

 hatcheri the outer valley does not get in between the longitudinal 

 expansions mentioned. The writer has seen relatively few speci- 

 mens of the domestic horse in which the outer valley fails to push 

 in between the longitudinal valleys of the molars, and a specimen is 

 found now and then in which the condition is found in the premolars 

 which prevails in the molars. 



In E. caballus the outer valley of the lower cheek-teeth has, in both 

 premolars and molars, a distinct reentering fold in its hinder border. 

 In E. hatcheri this fold is absent or small in the premolars and small 

 in the molars. 



In the upper cheek-teeth of E. niobrarensis the anterior inner 

 valley sends forward and outward, behind the deep reentering loop, 

 a prolongation which reaches the center of the tooth. This great 

 inner valley is quite different from that of E. hatcheri. In none of 

 the teeth of E. niobrarensis is there found in the anterior lake a 

 double M-like fold facing the head of the anterior inner valley. In 



1 Nova Acta. Acad. Caes. Leop. Car., vol. 52, p. 264. 



