16 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.63. 



least, on the incisors of Primates is a relatively ancient modification, 

 reaching back to an early primitive stage. 



Affinities of the Nothodedids. — As already mentioned, the fact is well 

 known that in the study of the early Tertiary mammals one frequently 

 meets with the closest similarity in general molar construction in 

 totally unrelated species. For this reason it is sometimes impos- 

 sible to assign a species even to its proper order on molar characters 

 alone, and frequently the knowledge of other anatomical features is 

 necessary to confirm actual relationships indicated by dental char- 

 acters. Matthew, in his discussion of the affinities of Nothodectes, 

 recognizing this fact, has stated that "as to the original reference of 

 the Plesiadapidae I am, as previously, disposed to reserve final deci- 

 sion until the skull and skeleton characters are more fully known,"' 

 and while admitting the "strong evidence" of the marked resem- 

 blance of the cheek teeth to those of the Eocene Primates, the char- 

 acters upon which Stehlin based his conclusion that Plesiadapis is a 

 true primate, did not consider the similarity of molar structure as a 

 strong argument on the ground that "various other Eocene mammals 

 which are certainly or probably not Primates equally resemble them 

 in molar construction." Nevertheless, I am inclined to think that 

 in his characteristically cautious manner, and with a very commend- 

 able desire not to go beyond what substantiated facts clearly war- 

 rant, Matthew has somewhat exaggerated the difficulty in distinguish- 

 ing the teeth of early Primates from those of unrelated forms. Espe- 

 cially does this seem true of certain forms where combined characters 

 of upper and lower dentitions may be considered. For example, in my 

 own experience I know of no species, certainly not primate, in which 

 is found the peculiar combination of modifications described above 

 (p. 8) as characteristic of Paromomys, and which is observed also in 

 the Notharctidae and in some at least of the Eocene Tarsiidae. 

 These modifications include for the lower molars a broad basined 

 heel and narrower, more or less forwardly sloping trigonid in which 

 the paraconid is progressively diminishing, or absent, its function 

 being taken over by the anterior cingulum ridge continuing with the 

 anterior flank of the protoconid to form a trigonid basin similar to 

 but smaller and more elevated in position than the talonid depression; 

 while the correlated modifications of the upper molars are a shallow 

 anterior basin external to the protocone, which basin and cone func- 

 tion with the talonid portion of the corresponding lower molar, and 

 a posterior basin, somewhat higher in position, formed by the poste- 

 rior cingulum ridge continuing with the backwardly expanded border 

 of the protocone to the summit of that cusp. The latter basin and 

 ridge function with the trigonid of the lower molar next behind. 

 This peculiar structure of the upper molars at least, while apparently 



•Bull. Amer. Mus. Nat. Hist., vol. 37, 1917, p. 837. 



