CHAPTER VI 



TOXODONTIA 



The toxodonts of the Deseado are much more varied 

 than those of the Santa Cruz, and less so than those of 

 the Casamayor; the teeth less hypsodont than in the Santa 

 Cruz, and more hypsodont than in the Casamayor; are 

 smaller than those of the Santa Cruz, and larger than those 

 from the Casamayor. It is a group of heavy, short-limbed, 

 nonadaptive animals, which, as time and competition pro- 

 gressed, gradually diminished in numbers and variety. 



The ancestral type must be sought in some such a form 

 as Henricosbornia, where the upper molars are brachydont, 

 have the four primary cusps distinct, and the connecting 

 crests of small size, and a cingulum moderately developed 

 on the front and rear sides. Progress is in the line of en- 

 larging the crests, so that, in the later forms, the two exter- 

 nal cusps are united to make a wall; and the anterior 

 external and the anterior internal cusps are united into the 

 large anterior lobe; while the posterior external and the 

 posterior internal cusps unite to make the posterior lobe. 

 These may remain relatively simple as in Rhynchippidae* ; 

 or with this simple arrangement of the cusps, the cingulum 

 may be developed into a platform around the anterior, 

 inner, and posterior sides of the molars, as in the Isotem- 

 nidae; or, with relatively simple molars, the incisors may 

 be specialized into caniniform-like teeth as in the Leon- 

 tiniidae; or secondary processes (or cristae) may develop 

 from the wall, making the complicated teeth characteristic 

 of Nesodontidae. 



* I have abandoned the family term Notohippidae, as the genus used as a 

 basis is very little known, and the forms Ameghino assigns to the family, to 

 my mind, mostly belong with the Nesodontidae. 



