LOPHIODONTID^. 617 



Total number of well determined species, one hundred and ninety-two. 



From the preceding table it can be readily seen that this order was 

 abundantly i-epresented during the Eocene period, and that the recent 

 species are comparatively few. It may also be observed that certain fami- 

 lies predominated during certain periods. Thus the prevalent Perissodactyla 

 of the Eocene are LopModontidce and Chalicotheriidoe ; those of the Miocene 

 are Rhinocerontidce and Palceotheriidoe. The Tapiridce and Equidce charac- 

 terize the latest Tertiary epochs. A genealogical tree of the order may be 

 constructed as follows: 



Equidse. 



Ehinoceridae. Palseotlieriidfe, 



Hyracodontidse. / Tapiridfe, 



Triplopidse. 



LophiodontidiB. 



Hyracotheriinfe. 



The types of the LopJiiodontidce and Chalicotheriidce differ only in the 

 two points of the separation or non-separation of the exterior crescents of 

 the superior molars, as ah-eady pointed out. That no great modification of 

 known forms (as JEdocium in the CJudicotheriidce, and Hyracotherium in the 

 Lophiodontidos;)vf owldihQ necessary to obliterate this difference, is quite clear. 

 The parent types of the order, which present the most generalized dentition, 

 Si/stemodon and Edocium, were cotemporaries of Lower Eocene age, 



LOPHIODONTID^. 



This family embi'aces a larger number of known species than any of 

 the others of the order. With one exception all the species belong to the 



