AUT. 1. PRIMATES OF THE FORT UNION GIDLEY. 15 



recurved crown on which is developed a posterior basal heel com- 

 parable to that observed in the Nothodectids. But this general 

 resemblance seems to me to be only structural parallelism. In favor 

 of the hypothetical reference of this tooth to one of the small Multi- 

 tuberculata, is the fact that, although not corresponding very closely 

 with any known incisors of this group, it approaches the modification 

 observed in certain Multituberculates in just the feature in which it 

 differs the most from the Nothodectid type, namely, in that the 

 accessory cusps are all on one side of the main cusp of the tooth 

 crown, instead of the more symmetrical arrangement seen in Notho- 

 dectes. 



The other two incisors referred to above more nearly conform to 

 the Nothodectid type. One of these (No. 9928), in which there are 

 five instead of three cuspules, has a strongly developed, blunt-heel 

 cusp, a relatively short, posteriorly concave crown composed of a 

 main slightly elongate cusp, with two closely placed cuspules on its 

 inner flank, and two, more widely spaced ones on its outer side. 

 The other tooth (No. 10010) is shorter crowned than the one just 

 described, and the heel cusp is very incipiently developed. It has a 

 single cuspule on either flank of the main cusp as in the Nothodectid 

 incisor, but the flanking cuspules, instead of being rounded in cross 

 section, are greatly expanded to more completely inclose the poste- 

 rior basin of the crown. The inner cuspule, moreover, is situated 

 much nearer the summit of the crown than is the outer one. In 

 Noihodectes the flanking cuspules are of nearly equal height. It is 

 possible that one or both of the last two teeth just described may be 

 upper incisors of some species of the Tarsiid group described above. 



The occurrence in this collection of so many incisors presenting 

 variations of the general modification typified in Nothodectes and 

 Pronothodectes is significant, and suggests that this development is a 

 primate peculiarity which was especially emphasized in the Plesia- 

 dapidae with their precociously developed incisors. That the Pri- 

 mates were susceptible to this general type of modification has 

 recently been pointed out to me by my colleague in anatomical 

 research, Dr. Ale§ Hrdlicka, who has recently made some extended 

 studies and interesting discoveries in connection with certain phases 

 of development of the human teeth. He has found that, not only in 

 man, but in many of the lower anthropoids, the incisor teeth, both 

 upper and lower, frequently have the crown definitely divided into 

 three or more cusps or lobes, a main median and two lateral ones, 

 in varying proportions as to size, associated with two other flanking 

 ridges, and a basal heel cusp. On examining embryos he frequently 

 finds this heel cusp calcifying from a separate center from the 

 remainder of the crown, with which it often very imperfectly fuses. 

 This would seem to indicate that the acquirement of a heel cusp, at 



