Gregory: notharctus, an American eocene primate 



129 



buccal and lingual branches. There is a rapid increase in size and especially in transverse diameter as 

 we pass from p^ to p^ p^ being a very small tooth, while p* in progressive species is nearly as wide as m^ 



This general description of the premolars and their roots applies equally well not only to Pelycodus 

 but to all the most primitive Eocene carnivores and ungulates. Pelycodus and Notharctus thus retained 

 a very primitive general plan of the premolars. 



In Adapis yarisiensis, the whole premolar series is evidently more specialized than in the more primi- 

 tive forms of the Notharctiniae and in a somewhat different direction; for, while p^ has become almost 

 molariform, p'^, p", p"^ are becoming like each other and like the canine, all having a compressed, sharp- 

 edged crown wdth a single tip and a strong internal cingulum. In Adapis magyius, p^ has likewise become 

 submolariform, but there is much more of the primitive difference between p'^, p^, and p\ this species 

 being less specialized in this respect as in many others. In general, the lower premolars conform in 

 appropriate manner to the above described characters of the upper premolars. Adapis rutvmeyeri (Stehlin, 

 1916, Taf. XXI.) is even more primitive than A. magnus, but already shows some disposition for po, ps 

 to become like each other and different from p4. 



First Upper Premolar 

 Text Fig. 35 



More in detail, p^ is known in very few specimens of the Notharctinte. In the type of A'', oshorni 

 it is a very small simple tooth with an asymmetrical crown, strongly convex externally, with a short 

 anterior ridge (crista anterior), a conical tip, a long posterior ridge (crista posterior) and an oblic^uely 

 sweeping cingulum, which is confluent in front with the crista anterior and in the rear with the crista 

 posterior; on the anterolingual face there is a very faint suggestion of a flattening or concavity, serially 

 homologous with the anterolingual face of the paracone of succeeding teeth. It is well separated both 

 from the canine and from p- b^- eciual diastemata of about one millimeter in length. This equal spacing 

 of the first premolar is found also in the lower jaw in N'otharctus tyrannus, N. venticolus, N. nunienus, 

 Pelycodus relictus and apparently also in the primitive P. triyonodus, but not in P. ralstoni. It may be 

 reminiscent of a primitive condition in which the simple conical upper premolars were separated by equal 

 intervals for the accommodation of the simple lower teeth. In connection wdth the enlargement of the 

 premolars in the later species the diastemata are more or less abbreviated, that between pi and po and p\ 

 p^ being lost in A^. pugnax and A^. crassus; the large size of the upper canines, however, necessitates the 

 retention of the diastema in front of pi. 



in A^. oshorni does not articulate Avith pi, its anterior edge being some distance behind that tooth 

 when the jaw is closed; but its posterolingual face shears past the anterolabial surface of po after the 

 fashion of the homologous surface of the succeeding premolars. The concavities on the lingual surfaces 

 of p\ p", c, i'^, i^ may all be adapted rather to the surface of the tongue, which in modern lemurs presses 

 against them, than to the action of the opposing lower teeth. 



In Adapis parisiensis (Stehlin, p. 1168) p^ is more compressed, with sharp anterior and i^osterior 

 cristae. In Adapis magnus (p. 1254) it is very small and less compressed; in either case its form is readily 

 derivable from the more primitive type preserved in Notharctus. 



First Lower Premolar 

 Plates XXXVI, XXXVII 



The first lower premolar (pi) is somewhat similar to p^ but more compressed; it shows in a very 

 incipient stage the tendency to become differentiated into an anterior moiety, or trigonid, and a posterior 



