GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



135 



tion with the parastyle; it is thus homologous with the protoloph of the molars. Behind the protoloph 

 and incipient protoconule there is a small depression corresponding with the central fossa of the molar 

 crown and guarded posteriorly by a low transverse papillate ridge which foreshadows the metaconule. 

 Behind the ridge in question and on a lower level lies the posterior cingulum. 



In Pelycodus trigonodus (Amer. Mus. Nos. 15017, 16089, etc.) p'* is not materially different from that 

 of P. ralstoni, save that the protocone is perhaps a little larger. Old individuals show the tip of the para- 

 cone worn by the hypoconid of p4, the tip of the protocone worn by the talonid of p4, and a worn ovoid 

 area in the fossa between the protocone and the posterior cingulum; this was probably worn by the promi- 

 nent paraconid of mi. In an old individual of Nothardus venticolus (Amer. Mus. No. 14655) the center 

 of the ovoid area above mentioned is worn through so as to expose the dentine, which appears as a sub- 

 circular spot surrounded by a worn strip of enamel; so that the whole assumes the appearance of a worn 

 down " tetartocone " or pseudohypocone, like that of the molars. In Nothardus nuniemis (Amer. Mus. 

 No. 4735), from the Wind River formation, the slightly worn tip of the paracone is just beginning to 

 become constricted on its posterior ridge and to give rise to an incipient metacone. This indication 

 is confirmed by the presence of a very faint groove running down on the lingual slope of the paracone 

 from the point of cleavage of the para- and metacones; this groove is the beginning of the external valley 

 between the paracone and the metacone. In Nothardus matthewi, a conservative species of the Lower 

 Bridger, there is extremely little suggestion of the cleavage of the para- and metacones but in all the other 

 Lower Bridger species this separation is more or less marked, especially in N. pugnax (Amer. Mus. No. 

 14567) where the cleft and groove are very pronounced, as they are also in the immediately succeeding 

 species N. crassus of the Upper Bridger. In these later types p"* becomes nearly as wide transversely as 

 the molars, the protocone widens anteroposteriorly and the middle of the crown loses the constricted 

 appearance of the earlier types, the anterior and posterior borders becoming more nearly parallel. But 

 even at this stage p^ is by no means molariform, although all the principal elements of the molar crown 

 are more or less distinctly indicated especially in the worn condition. 



In Adapts parisiensis p^ is very nearly molariform, having two widely separated external cusps and 

 a sharp oblique protoloph (protocone-parastyle ridge) ; it has, however, no hypocone. It is more elongate 

 anteroposteriorly than that of the Notharctinse and these two have evidently evolved along quite divergent 

 paths. In Adapis niagnus p"* is less molariform than that of A. parisiensis; it is wider transversely and 

 less elongate anteroposteriorly, but the subfamily affiliation is evident. A more primitive stage of p"* is 

 shown in A. riitimeyeri (Stehlin, 1916, Taf. xxi, figs. 27, 29) but even in this p"* has two very distinct 

 external cusps, which are much more distinct and separate than those even of the latest of the Notharc- 

 tinse. In Adapis sciureus p* is not known, but the very retarded condition of the hypoconid of p4 gives 

 reason to believe that p^ either had but one external cusp or that the metacone was connate with the 

 paracone. 



Fourth Lower Premolar 

 Plates XXXVI-XLI; Text Fig. .38 



P4 in its most primitive form is found in Pelycodus ralstoni and P. trigonodus, where the cusps of 

 the incipient trigonid have the sharp pricking character which was so common among Paleocene and 

 Eocene insectivorous mammals. 



P4 is distinguished from ps by the following characters: it has a distinct paraconid (at the front end 

 of the crista anterior); the large thorn-like metaconid is an outgrowth of the "metaconid ridge" which 



