178 



EVOLUTION OF MAMMALIAN MOLAR TEETH 



de pi '. pi P r 



de 



mc a 



FIG. 166. Upper and lower cheek teeth of several Lower Eocene Equidse. All x 1. 



First or upper jiiiun . Upper cheek teeth of Eoltippvs (Protorohippus) venticolus, Wind River 

 Formation, Lower Eocene, Stage II. Side view. 



Second figure. Crown view of the same. 



Third figure. Lower cheek teeth of Eohippus cristatus, Wasatch Formation, Lower Eocene, 

 Stage I. 



Fourth (Join rfiiii'n ). Lower teeth of Ef.ihiiniv.it index, Wasatch Formation, Lower Eocene. 



All figures natural size. After Wortman. 



Observe: (1) The general similarity in the molar pattern to that of the Eocene Tapiridie, as 

 shown in the development of two transverse ridges in the upper and lower teeth, a feature 

 destined to be emphasized in the Tapiridre but highly modified in the Equidse. (Of. Figs. 100- 

 164, 172-174.) (2) The incipient character of the mesostyle (/<(.<.), and the appearance of the 

 hypostyle (As.) in ii-m s . (3) The complication of the fourth premolar pi, by the development 

 of a protoloph from the protoconule and deuterocone, and of a small metaloph from the 

 metacouule, whereas in p :i , the protoloph is developed from the protoconule only, the meta- 

 loph from the deuterocone. (4) In the lower molars note (1) the molarization of p 4 , (2) the 

 development of an oblique crest running from the hypoconid (/<?/<*.) to the metaconid, (3) the 

 faintly incipient twinning of the metaconid ; all three features being progressively developed in 

 the later Horses. 



Fio. 167. Upper cheek teeth of Lambdotherium popoagicum, from the Wind River Formation. 

 Lower Eocene, a primitive Titanothere with molars of tritubercular derivation, x \ . Observe 

 the subcrescentic character of the protocone,the continuity of the metaconule with the hypocone. 



