74 ON THE HOMOLOGIES AND ORIGIN OF THE 



crests represent the modification of opposite tubercles, and those where the tuber- 

 cular elements of the crown are alternate. These series may be called the Amoebo- 

 dont (alternate), and Antiodont (opposite), and the component types are— 



Antiodonts. 

 Selenodont (Euminants); Tapirodont {Tapir, Rhinflcerus); Trichechodont (manati, 

 Elephant). 



Amcebodonts. 

 Palaotheriodont (Palseotherium, etc.); Symhorodont (Palseosyops Symborodon, 

 etc.) ; Bathmodont (Bathmodon, etc.), Loxolophodont (Uintatherium, etc.). 

 These types are defined as follows : — 



1. Antiodonts. 

 Selenodonts. 



The tubercles separate or united at their angles, much elevated, narrow cres- 

 centic in section, separated by deep valleys. 



To this group belong the molars of the Ruminants, the TraguUd(S, the Oreo- 

 dontid(2, and Merycopotamida ; of Perissodactyles the Equidce. 



Tapirodonts. 



The outer tubercles longitudinally compressed, subcrescentic in section; the 

 inner transversely compressed, continued as transverse ridges to the end or middle 

 of the corresponding exterior crests. 



Rhinoceros, Tapirus, Hyrachyus, Lophiodon, and Hyrax represent this type ; the 

 last molar of Lophiodon appears to be Amoebodont. 



Trichechodonts. 

 Tubercles confluent into two or more transverse crests. 



The Dinotherium represents this form, so does Elephas, Trichecus (the manati), 

 and a number of the larger Marsupialia. 



2. Amcebodonts. 

 PalcBOtheriodonts. 



External tubercles longitudinal, subcrescentic in section; the inner united with 

 them by transverse oblique crests. 



Palceotherium and Anchitherium present this type, which only differs from the 

 Tapirodont in the alternation of the opposing tubercles. 



