88 ROYAL SOCIETY OF CANADA 



in the posterior half of its len^h; (7) the diastema in advance of p^ is 

 proportionately longer and its margin is not so arched. 



For the Cypress hills species of Hyracodon, the name priscidens 

 is proposed, indicative of the less advanced stage of its dentition as com- 

 pared with H. nebrascensis. In the form of its premolars, it is decidedly 

 primitive, and implies a position in a direct line of descent from 

 Hyrachyns. A progressive character is seen in the squareness of the 

 premolars, which, in Hyrachyns, 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 deuterocone 

 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 of the crown, lengthening the anterior loph and also effecting a 

 juTiction 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 p^ 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 imion 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, pre- 

 sents a concave surface forward. A variation is noticed in the right 

 first premolar of the Cypress hills specimen. In this tooth the sinns, 

 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 



