41 



other premolars in this figure denote a stage of evolution much in advance of the correspond- 

 ing teeth of H. priscidens. 



In the premolars of H. priscidens the deuterocone arises from the protoloph, as in p^ and 

 p4 of Hyrachyus agrarius, Leidy, of the Bridger Eocene of Wyoming and Utah ; also, in the 

 second, third and fourth premolars, the general outline of the tooth is quadrangular instead 

 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 

 tritocone ; 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 protoloph 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 decided tubercle, there is a delicate crochet* which is of interest 

 as a decidedly progressive character. In the molors 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 extend, as in the other molars, far posterior to its junction 

 with the metaloph, but is curtailed at this point, in consequence of which there is only a slight 

 indication of the formation of a posterior valley (postsinus) that in H. nebrascensis has reached 

 a more advanced stage. 



The order of premolar transformation in H. priscidens is apparently an exception to the 

 usual metamorphosis of the Hyracodont premolars, which, as stated by Osborn in his memoir 

 on " The Extinct Rhinoceroses" 1898, p. 90, is presented in three successive stages of evolu- 

 tion toward the molar pattern in the second, third and fourth premolars, the last premolar 

 (p4) being the most advanced. 



In Jf. priscidens the fourth premolar is the least advanced, as regards the relation of the 

 lophs to each other, although in other respects, viz., in the presence of secondary crest folds 

 ("crista," "antecrochet" and "crochet "), a decided advance has been made, and it may be 

 considered in this regard as more progressive than pi^, p'^ and p^. 



* 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." 



12529—6 



