72 EVOLUTION OF MAMMALIAN MOLAR TEETH 



the same Mesozoic source, it is important to unify our methods of 

 description by adopting a set of terms which refer back to the primitive 

 form and position in place of those which were based upon the com- 

 paratively modern form and position. 



o. 



The above discussion of the homologies of the molar elements of 

 ungulates was followed in 1890 by a contribution which may be 

 entitled : 



APPLICATION OF THE THEORY OF TRITUBEP.CULY TO THE 



PERISSODACTYLA. 



(Scott, W. B., and H. F. Osborn, "Preliminary Account of the Fossil Mammals 

 from the White River and Loup Fork Formations contained in the Museum of Comparative 

 Zoology. . . . The Perissodactyla," by Henry Fairfield Osborn. Bull. Mus. Comp. Zool. XX., 

 No. 3,1890, pp. 88-91.) 



The Horse Molars. 



The upper molars of Mesohippus [Fig. 161rfJ clearly show the first step 

 in the formation of the posterior pillar, pp., which is so conspicuous a 

 feature in Anchitherium, in the posterior valley. This can also be observed 

 in a still simpler stage in a specimen of Anckilophus, from the French 

 Phosphorites. Step by step with the development of this cusp appears 

 the posterior pillar, p, in the lower molars, behind the entoconid ; this 

 accessory cusp can be traced back to the teeth of Epiliippus. When it 

 finally unites with the entoconid, in Hipparion, it forms the posterior twin 

 cusp (b, b, Elitimeyer), which is analogous to the anterior pair formed by 

 the union of the metaconid and anterior pillar a (a, a, Eiitimeyer). 



Thus the transition from the Mesohippus to the Anchitherium molars 

 is very gradual, as shown in the accompanying figures. By tracing back 

 the rise of the eleven elements which compose the upper Equus molar, we 

 find that six belong to the primitive sexitubercular bimodont crown. Two 

 elements of the ectoloph, the anterior pillar and median pillar, rise from 

 the simple primitive basal cingulum of the Hj/racotherium molar ; the 

 same mode of development, we have just seen, is true of the posterior 

 pillar. The eleventh element, the fold of the postero-external angle of 

 the crown, p, is not prominent until we reach Equus. The term 

 " posterior pillar " is taken from Lydekker ; the other terms, " median " 

 and " anterior," are applied to parts which have an analogous origin from 

 the basal cingulum. The remaining coronal cusps are readily identified 

 with their homologues in the primitive tritubercular molar. 



