250 THE STEUOTURAL EVIDENCE OF EVOLUTION. 



sent an alternate crested type. This is not constituted as in the 

 next form, by a union of alternating tubercles, but as in the Sele- 

 nodonts by tbe special development of each crest into a ci'escent 

 extended antero-posteriorly. As alternating, the inner crescents 

 stand at the apices of the outer, and are connected with them. 

 In Anchitherium the inner are so reduced as to constitute a condi- 

 tion intermediate between the Hippodont and Symhorodont types. 



Symborodonts. — The alternating tubercles connecting by 

 oblique ridges which form together two Vs. 



To this type is to be referred a great number of Perissodac- 

 tyles, e. g., AncliWiejHum, Palceotherium, Palceosyops, ilenodus, 

 Symhorodon, AncMppodus. 



Loxolopliodonts. — Alternate cusps connected by two cross 

 ridges from the outside forward and one from the inside forward ; 

 from which result an oblique posterior cross ridge, and a V open- 

 ing inward. 



Here are Eobasileus and Bathmodon ; the last molar of the 

 latter having the anterior ridge of the V quite low. 



S. Comparison of the Opposing Series. 



In review, the above types of molar dentition maybe classified 

 as follows, with reference to the amount or complication of the 

 modification of the tubercular type. The orders of Ungulata with 

 which they correspond, are also given. 



a. Both inner and outer tubercles crescentoid. 



( Ruminantia. 

 Selenodonts. j {Anoplothcrium, Oreodon, Hyopotamus). 



( Perissodactyla {Equidce). 



p. External tubercles only crescentoid ; the inner transverse 

 or tubercular. 

 Tapirodonts. j 



Palaeotheriodonts. V Perissodactyla in gen. 

 Symborodonts. ) 



y. Neither kind of tubercles crescentoid, but united in pairs. 

 Trichecodonts. Prohoscidia. 



It may be added that the groups arranged under y are the 

 only ones in which the types of crests of the superior and inferior 

 molars are fundamentally simple and alike. Thus in the group 

 a, tubercles of both upper and lower series are modified independ- 

 ently to produce tlie type ; in group /3, the tubercles of the upper 

 series are modified independently of each other, while those of the 



