8 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.63. 



P. alticuspis only in being proportionately wider, and in having the 

 summit of the somewhat higher protocone relatively more nearly 

 approached to the paracone. 



Most of the differences noted above suggest for the species just de- 

 scribed a shghtly different line, or direction, of development than is 

 indicated in P. alticuspw. It is possible, therefore, that more com- 

 plete materials may prove that these two species do not form a 

 natural generic group. However, for the present they may be treated 

 as such. 



RELATIONSHIPS OF THE GENERA PAROMOMYS AND PALAECHTHON. 



In the species of genera just described, the molar teeth seem in 

 certain respects to suggest relationship to the Notharctidae. This is 

 especially true of the modifications of mg and the position and mode 

 of progressive disappearance of the paraconid in all the molars of the 

 Paromomys group. Also there is a significant resemblance in the 

 form and manner of development on the trigonids of the lower molars 

 of an anterior ridge or shelf which takes over the function of the 

 paraconid. This adaptive feature is directly associated with the 

 development in the upper molars of a posterior basal expansion of 

 the protocone, and the forming of a posterior basin, as already de- 

 scribed (p. 5). This is similar to the condition observed in the species 

 of Pelycodus which have not yet arrived at the stage where the true 

 hypocone appears. It is also a feature of Phenacolemur of the 

 Apatemyidae. In fact, this peculiar development of the upper and 

 lower cheek teeth, apparently constitutes a distinctively primate 

 characteristic, which, while not found in all families of the order, 

 seems to have been repeated over and over again, with slight vari- 

 ations, in several related or unrelated groups, and, so far as I am 

 aware, is not found in any other order of mammals. (This is, of 

 course, assuming that the Plesiadapidae are true Primates.) It 

 therefore follows that the apparent likeness of Paromomys to the 

 Notharctidae, suggested by similarities of the molar teeth, may or 

 may not denote relationship to this group and seems to be more 

 than outweighed by the important differences observed in the mod- 

 ifications of the anterior teeth. Thus the reduction in Paromomys 

 and Palaechthon of the premolar series to three or two, and the more 

 advanced specialization and enlargement of a single pair of incisors 

 in the lower jaws are far more important features than the molar 

 resemblance, and seem to preclude the possibihty of a near or at least 

 ancestral afiinity of the group to the Notharctinae. Also in this 

 group the orbit apparently is of the enlarged type as in the Tarsiidae. 



The molars of this group, in some respects also resemble those of 

 the Nothodectids. However, the very marked difference of modifica- 



