144 BULLETIlSr 16 9, UNITED STATES NATIONAL, MUSEUM 



anterior teeth followed by three premolars, but in it there is a small 

 tooth between these two enlarged teeth, which is either much more 

 reduced or wholly absent in the much older Fort Union genera. 

 Absarokius may very closely resemble the Fort Union genera in the 

 anterior teeth, although this is very dubious, as they are known in 

 Absarokius only from poorly preserved alveoli of one specimen, which 

 seems to show less disparity between the incisor and the ?canine. 

 Tetonius and the European Necrolemur have a single enlarged anterior 

 tooth,^^ a condition that could be derived from that of the middle 

 Paleocene genera, although there is no adequate evidence that it was 

 so derived. 



P4 is more primitive in Palenochtha than in any later tarsioid genus 

 known to me, but the difference in such forms as Anaptomorphus or 

 ^'Omomys" belgicus is not marked, and as the increasing and diverg- 

 ing specialization is in keeping with the relative ages it has no crucial 

 bearing on general affinities. The lower molars of Palenochtha are 

 much like those of Wmomys vespertinus, "Omomys" belgicus, and simi- 

 lar forms, that is, those Eocene tarsioids in which the molars are least 

 specialized. The same may be said of the upper molars: those of 

 Palenochtha show distinctions by which the genus may be recognized 

 (such as the internal groove and more inclined protocone), but they 

 very closely resemble the least aberrant Eocene tarsioids. Compari- 

 son with Wmomys vespertinus is especially suggestive of affinity. Most 

 later genera differ in the manifestly progressive development of sec- 

 ondary internal cusps. 



It is, incidentally, worthy of note that Palenochtha has no known 

 character that would exclude it from ancestry to Tarsius, itself, 

 although of course the absence of intermediate stages makes this ob- 

 servation unworthy of being advanced except as an interesting but 

 wholly untested possibility. 



The more advanced P4 of Palaechthon does not call for detailed con- 

 sideration. It is in line with progressive changes in many tarsioids 

 ■and some other primates, although it should be noted that the devel- 

 opment of P4 in the Tetonius, Carp>olestes, Apatemys, Plesiadapis, and 

 some other groups lies along distinctly different lines. 



The peculiar molar structure suggested in Palaechthon and fully 

 developed in Paromomys is more distinctive. The short quadrate 

 trigonid and marginal paraconid approximated to the metaconid 

 appear among tarsioids in only one or two later genera. Absarokius 

 has very similar trigonids on M2-3, but that of Mi is more elongate, 

 probably secondarily in connection with the shearing development of 

 P4. Other American tarsioids are more distinctive. Among European 

 forms, only Necrolemur and Microchoerus are similar, and they are 



61 Commonly called the canine, but it seems to me more probable that it is an incisor in both cases. 



