40 



is relatively much shorter, with a concomitant greater development of the metaloph, (4) the 

 exterior cingulum is developed only on the posterior half of the base of the ectoloph in the 

 seven teeth, and the internal cingulum is absent in p\ (5) the parastyle in the premolars is 

 only slightly developed, (6) the skull is apparently flatter and relatively more elongate, the 

 lower margin of the orbit being less distant from the alveolar border, and the jugal less curved 

 upward in the posterior halt of its length, (7) the diastema in advance of p^ is proportion- 

 ately longer and its margin is not so arched. 



For this species of Hyracodon the name priscidens has been proposed, indicative of the 

 less advanced stage of its dentition as compared with H. nebrascensis. In the form of its 

 premolars it is decidedly primitive, and implies a position in a direct line of descent from 

 Hyrachyus. A progressive character is seen in the squareness of the premolars which in 

 Hyrachyus are triangular; also in these teeth the metaloph is much advanced in comparison 

 with the Eocene genus. 



In H. priscidens, as in the type species, m^ is much the largest tooth, and m^^ is larger 

 than p*. The molars occupy about the same space antero-posteriorly as the premolars. 



In the premolars there is a progression toward the molar pattern, but the advance has 

 been slow. The anterior premolars are more progressive than the posterior ones in some 

 respects. This is shown in the tendency to the separation of the tetartocone from the deutero- 

 cone seen in passing from p* forward. In p* the tetartocone and the deuterocone are very 

 closely united, but in p^, although still connected to the protoloph, the tetartocone has moved 

 farther toward the posterior border ot the crown, lengthening the anterior loph, and also 

 effecting a junction with the metaloph. 



In p* the cross lophs are unequal in length, the protoloph, in which the tetartocone is 

 very intimately united with the deuterocone, not passing beyond a point in line with the 

 inner end of the metaloph, which is short and curves slightly backward. In p^ the protoloph 

 is increased in length by the shifting backward of the tetartocone, with a tendency to separate 

 from the deuterocone. The two lophs remain distinct, the anterior one passing slightly 

 beyond the inner end of the metaloph, which in this tooth is developed to about the same 

 extent as that of p^, with a like backward obliquity. In p2 the protoloph is still further 

 increased in length, and curves round the inner margin of the crown considerably past the 

 metaloph toward the posterior border of the tooth. The metaloph curves slightly forward 

 and unites with the protoloph at a point some distance in advance of the latter's posterior 

 termination. The increased length of the protoloph is due to the further recession of the 

 tetartocone from the deuterocone, although the union of the two remains complete. In p^ 

 the protoloph is separate from the metaloph, a narrow but distinct sinus dividing them, and 

 the tetartocone arises from the metaloph which in its inner half presents a concave surface 

 forward. A variation is noticed in the right first premolar of the Cypress Hills specimen. In 

 this tooth the sinus, seen in the left first premolar, in advance of the metaloph, does not occur, 

 in which case the tetartocone would still be said to arise from the protoloph which, com- 

 mencing at the ectoloph, behind the parastyle, forms a high continuous wall curving round 

 the inner border ot 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 qorresponding tooth of K nebrascensis as 

 figured by Leidy in plate XIV, figure 5, accompanying his description of the type species in 

 " The Ancient Fauna of Nebraska," 1852, (Smithsonian Contributions to Knowledge). The 



