134 GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



articulates with the anterior face of p^, the tip of the protoconid articulating with the parastyle of p^ 

 It will be shown below that one side of the upper and lower jaws could be pivoted upon ml and when 

 this happened ps could be swept transversely across p^ so as to produce the worn surface on the pos- 

 terior face of the former and on the anterior surface of the latter. Similar articulating relations of pi 

 are very well shown also in N. osborni (left side of type) and in N. crassus (Amer. Mus. No. 11982). 



In Adapis parisiensis ps was a much compressed and low-crowned, sharp-edged tooth, with sharp 

 internal and external cingula, a small talonid and a low metaconid ridge. It is much more specialized 

 than the corresponding tooth of the oldest members of the Notharctinse. In the later Notharctinse this 

 tooth has become widened transversely. In Adapis magnus ps is not as compressed as it is in A. parisiensis, 

 but is otherwise similar. In ps of Adapis sciureus (Stehlin, 1916, fig. on p. 1515) we have apparently 

 a more primitive form of the adapine type which is, however, obviously not as primitive as that of Pely- 

 codus trigonodus. In A. riitiyneyeri (Stehlin, 1916, Taf. xxi, fig. 24) ps is as little like p4 as it is in A. 

 magnus. It has a compressed oval crown somewhat obliquely placed, with a large protoconid, a stout 

 paraconid, no metaconid and a low hypoconid. 



Fourth Upper Premolar 

 Plates XXXV, XLI; Text Fig. 40 

 P* in the oldest and most primitive Notharctinse (P. ralstoni, Amer. Mus. No. 16089) has an undivided 

 pointed paracone with sharp anterior and posterior cristse; the external cingulum is barely continued 

 across the paracones; it rises into a distinct parastyle and an incipient metastyle; the protocone spur is 



midd/e ofmeTa/oph/d 



mM/e ofcT/'s/a ob//qua 



shears pasf metacon id- 

 pro focon id shear ofm^ 



•tejva/ SeAveen m^&m^ 



/ope (worn) fom 

 pr. to he/iif7d mtcl 



B 



~-metacone , 

 paracone m 

 metaconemj protocone 

 centra/ /ossa m ^ 



Fig. 39. Diagram: occlusal relations of the right nr and nio in tlie most primiti\e species of Pclycodus. See page 149. 



A. Pelycodus ralstoni. Second right upper molar. Amer. Mus. No. 16089. X i'. 



B. Pelycodus ralstoni. Second right lower molar. Amer. Mus. No. 16093. X J. "Protocone m=" should read "protoconule m-." 



relatively narrower than in later types so that the contour of the crown is slightly constricted in the 

 middle between the paracone and the protocone. From the tip of the protocone runs outward and forward 

 a sharp low crest which, after giving rise to an incipient protoconule, is continued outward to its junc- 



