124 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



(N. veniicolm). The long axis of the crown inchnes gently toward the mid-line of the skull; it measures 

 2.2 mm. in this direction and 2 mm. in thickness. Its root is subcylindrical and less procumbent than 

 that of i^ It is thus less specialized than that of Adapis, which has become more like the wide-edged i'. 

 It was separated by well-marked diastemata from both the central incisor and the canine, whereas in 

 Adapis (according to Stehlin's figures) the widened crown of i" was nearly or cjuite in contact both with 

 the central incisor and with the canine. So far as one can judge from imperfect material the opposite 

 incisor series of Notharclus were somewhat less inclined toward each other than was the case in Adapis, 

 especially Adapis magnus. 



A second upper incisor of a species of Pelycodus (Amer. Mus. No. 15034) from the Wasatch formation 

 is much like that of Nothardus; the single-fanged canine also resembles that of the female of Notharclus 

 oshorni, save that it is less robust. Taken in connection with other evidence this indicates that the 

 general construction of the muzzle and of the dentition even in the most primitive species of Pelycodus 

 was not dissimilar to that of Notharctus. 



The loiver incisors are all preserved //( situ in the paratype of Notharctus pugnax (Amer. Mus. No. 

 11480); the lower incisors of one side are preserved in the type of rostratus (Amer. Mus. No. 5009) 

 and in N. venticolus (Amer. Mus. No. 14655) ; in all these cases they are much more worn than the pre- 

 molars, this suggesting that they erupted very early, along with the molars, as in Adapis; the only little- 

 worn lower incisors known to the writer are in the type jaw of A^. (" IViinolestes'^) anceps Marsh in the 

 Yale Museum. These are gently procumbent with short spatulate crowns. The crown of ii is broadly 

 spatulate, truncate at the tip, 2.() mm. in width; that of u is obliquely spatulate, width 3.2, with a round 

 edge. The lateral tips of the crowns of ii and l. were in contact. The much worn lower incisors of N. 

 pugnax, type, are all separated from each other by diastemata of somewhat less than a millimeter in 

 width, while the diastema between 1-2 and the canine is about 2 millimeters. The unworn crowns were 

 moderately expanded transversely and may have been in contact. In Adapis all the diastemata are 

 small and the wide flat crowns are nearly or quite in contact. (Cf. Stehlin, 1912, p. 1173, figs.) 



The central lower incisors (ij are moderately procumbent. The half-worn crown bears a transversely 

 extended worn edge, but the unworn crown probably has a bluntly pointed tip. The cutting edge articu- 

 lated with the chisel-like edge of i^ and was no doubt more or less rubbed by the under surface of the 

 tongue. In old animals the worn surfaces of the procumbent lower incisors face upward and forward, 

 showing that they were partly overhung by the upper incisors. 



The lateral lower incisors {l<) are much larger than ii; when well worn the crowns are oval antero- 

 posteriorly; when unworn they probably ended above in a rounded truncate tip rather than in a very 

 wide chisel-like edge; they articulated with the lingual surface of i". The slender roots of the central pair 

 of incisors extend about one-third the way down the length of the symphysis and are parallel and quite 

 near to it. The much stouter but short roots of the lateral incisors occupy the triangular area between 

 the roots of the canines and those of ii. 



From the construction and articulating relations of the upper and lower incisors and from the relative 

 size and positions of their roots in Notharctus I think it may be safely inferred that the third upper and 

 lower incisors of the complete Eutherian formula are the ones which are missing in Notharctus and in all 

 » other primates. Modern representatives of the eighteenth century "ecole des faits" may object that 

 until the Paleocene or earlier ancestors of the primates shall be discovered we cannot knoio that the 

 ancestral formula for the incisors was if. This matter will be discussed later, but meanwhile it may be 

 noted that the absence of i;i in the lower jaw is associated with (I do not say conditioned by) the enlarge- 

 ment of the canine, the procumbency of ii and the consequent restriction of the space for the roots of i2. 



