430 PROCEEDINGS OF THE ACADEMY OF [1892. 



VIII. PERISSODACTYLA. 



A very high degree of premolar differentiation is characteristic 

 of this group. In all of the existing fjxmilies these teeth have 

 assumed the molar pattern, a tendency which very early becomes 

 apparent in the ancestral forms, and even the extinct phyla, such as 

 the Hyracodontidce, Palceotkeriidce, Titanotheriidce, etc., run through 

 the same course. 



The Wasatch perissodactyls have all advanced beyond the Phen- 

 acodontidce which are contemporary with them, so far as dental evo- 

 lution is concerned. In Systemodon the only one of the superior 

 premolar series which is without additions to the protocone is the 

 first, and even in that there is a faint anterior thickening of the 

 cingulum, which represents the pillar or buttress, so conspicuous on 

 the other premolars. P^ has quite a large deuterocone and a rudimen- 

 tary tritocone which is barely separated from the protocone; the 

 anterior pillar is slightly better developed than on p^. Seen from 

 the external or buccal side, P^ and ^ resemble the molars, having 

 proto- and tritocones of nearly equal size, and the anterior pillar 

 largely developed ; internally, however, there is only one cusp, the 

 deuterocone, which sends out two crests toward the outer wall of 

 the tooth. These crests a2:)pear to be developed from conules analo- 

 gous to those of the molars, and in P^ the posterior conule is not 

 connected with either the deutero- or the tritocone. 



The inferior premolars vary somewhat in the different species of 

 the genus. In S. semihians p3~is very simple, having added only a 

 small metaconid to the compressed and acute protoconid. P^ has 

 also two internal cusps, the deuteroconid in front and tetartoconid 

 behind. The number of cusjds present is thus the same as in the molars 

 but the small size of the internal elements and the low heel give to 

 the crown a very different appearance. In S. tapirinus p*~ consists 

 of two crescent-shaped cusps, the proto- and metaconids, of which 

 the former rises considerably higher than the latter, but the tooth 

 possesses no internal elements at all. P^ has a similar crescent- 

 shaped metaconid, but the protoconid is compressed and pointed. 



The genus Isedolophus, which continues this series through the 

 Bridger and Uinta formations, does not present any modifications 

 in premolar structure which require special description. 



In the White River form Mesotapirns the last upper premolar 

 has assumed the molar pattern by the addition of a tetartocone and 

 in the lower jaw p^ and * have the same number of elements as the 



