[lambe] a new species OF HYRACODON 39 



the protoloph that, commencing at the ectoloph behind the parastyle, 

 forms a high continuous wall curving round the inner border of the 

 tooth for some distance past its union with the metaloph, giving to the 

 protoloph a length proportionately still greater than the corresponding 

 loph of p2. The left first premolar above described resembles the 

 corresponding tooth of H. nehrascensis as figured by Leidy in plate XIV, 

 figure 5, accompanying his description of the type species in " The 

 Ancient Fauna of îfebraska/' 1852 (Smithsonian Contributions to 

 Knowledge). The other premolars in this figure denote a stage of 

 evolution much in advance of the corresponding teeth of H. priscidens. 



In the premolars of the Cypress hills species the deuterocone arises 

 from the protoloph as in p^ and p* of Ilyrachyus agrarius, Leidy of the 

 Bridger Eocene of Wyoming and Utah, but in the second, third, and 

 fourth premolars the general outline of the tooth is quadrangular in- 

 stead of triangular as in Hyrachyus. The transverse diameter of p* 

 is relatively greater than that of either p^ or p2. The second, third, 

 and fourth premolars are provided with a well defined cingulum that 

 is continuous round the entire base of the crown, except at the base 

 of the tritoeone; at the base of the parastyle the cingulum is feebly 

 shown with increasing faintness in passing from p' to p-. In the first 

 premolar the exterior cingulum is developed only in the posterior half 

 of the ectoloph, the posterior cingulum is strong, the anterior cingulum 

 extends but a short distance from the parastyle and there is no 

 internal cingulum. 



In the molars the cross-lophs are nearly equal in length, the proto- 

 loph being slightly the longer, the hypocone is strongly developed and 

 of the size of the protocone, from which it is separated by a deep 

 anterior valley (medisinus). A crista, strongest in m^, is developed 

 from the ectoloph, and an antecrochet, of fair size in m^ smaller in m^, 

 and incipient in m^, is given off from the protoloph. In p*, in addition 

 to a small crista, and an indication of an antecrochet in the form of a 

 distinct tubercle, there is a delicate crochet,^ which is of interest as a 

 decidedly progressive character. In the molars there is no internal 

 cingulum, but posteriorly, anteriorly, and externally the cingulum is 

 as in the premolars, except that externally it is scarcely more than 

 suggested at the base of the parastyle. As already mentioned, the 

 ectoloph in m^ is short as compared with that of m^ and m^, principally 

 on account of the reduction in size of the metacone, which does not ex- 

 tend, as in the other molars, far posterior to its junction with the meta- 



' Professor H. F. Osborn, in his memoir on " The Extinct Rhinoceroses " 

 (Memoirs of the Amer. Mus. of Nat. Hist., Vol. I., Part III., p. 89, 1898), has 

 mentioned that the 'crochet ' is " peculiar to the true Rhinoceros molars " 

 and is " only feebly developed, if at all, in the Amynodonts and Hyracodonts." 



