rSpS.] Osborn, Evolution of the Aviblypoda. Part I. 1 73 



Evolution of the Molars in the Amblypoda. 



The upper and lower molars of the Periptychidse are purely 

 bunodont, characterized by a compressed trigon, the development 

 of accessory external cusps, and crescentic internal cusps upon 

 the superior premolars. The latter recall the teeth of Fanio- 

 lambda. Ectoconus is the only type in which the lower premolars 

 tend to become molariform. 



The Pantolambda superior molar (Fig. 2 A) shows a crescentic 

 disposition of the three primary cones, which are, however, closely 

 approximated as in the Periptychid^, the outer wall of the crown 

 extending widely into the parastyle. The lower molar shows a 

 lofty trigon distinguished by the marked elevation of the meta- 

 conid, and a talonid best shown in Fig. 2 D. No true hypoconulid 

 is developed. From the entoconid a spur extends forwards and 

 inwards, to which the designation ' entoconid 2 ' may be given, 

 as seen in Fig. 3. This little cusp be- 

 comes a very important feature of the 

 crown in Bathyopsis and Uintathcrium. 



The Coryphodon superior molar (Fig. 4), 

 as homologized by Cope, Earle and the 

 writer, exhibits a protoloph and an ecto- 

 loph consisting of a greatly reduced para- 

 cone, a vestigial mesostyle and strongly 

 crescentic metacone. In the last superior 

 molar these ectoloph elements are trans- 

 formed into a single oblique lophoid crest. 

 The inferior Coryphodon molar (Fig. 3) 

 shows a greatly elevated protoconid, an 

 enlarged metaconid and depressed para- 

 conid. The talonid consists of a hypo- 

 conid, entoconid and a low spur, the 

 entoconid 2, prophesied in Pantolambda. 



From this, the Bathyopsis and Uinta- 

 therium lower molar is readily derived, as 

 shown in Fig. 3, simply by the fission of 

 the metaconid into the metastylid, wA/, 

 and further reduction of the paraconid 

 and entoconid 2. This fission is an ad- 



Bathyopsis. 



pr' mo' 



Coryphodon. 



pr' rne' 



Pantolambda. 



Fig. 3. Lower molars, Am- 

 blypoda. Diagrams showing 

 evolution of crown. Not to 

 scale. 



prd protoconid ; J>ad para- 

 conid ; ;;/(' metaconid ; tnl^ 

 metastylid ; hyd hypoconid ; 

 end entoconid ; en. 2 ento- 

 conid 2. 



