TOXODONT CLASSIFICATION 89 



Rhynchippidae 



This family name is used for the three genera Rhyn- 

 chippus, Morphippus, and Eurygeniops, which made up a 

 part of Ameghino's family, Notohippidae. These forms I 

 find much simpler than Coresodon, Interhippus, Stilhippus, 

 and Nesohippus, which, by their molars, should be asso- 

 ciated with Nesodontidae, unless it should prove that they 

 did not have the incisors enlarged to caniniform teeth, in 

 which case another disposition will have to be made of 

 them. Ameghino places the Rhynchippidae among his 

 Hippoidea, leading to horses, but we found a nearly com- 

 plete skeleton of Rhynchippus equinus which in all particu- 

 lars is typically toxodont. In the Deseado we found 

 fourteen specimens belonging to this family, and strangely 

 enough they were all Rhynchippus, and all of the species 

 R. equinus. 



This family is distinguished by the brachydont, or 

 nearly brachydont, molar teeth, being relatively simple, 

 and the secondary cristae not being developed. The large 

 basin in the upper premolars and molars is, therefore, not 

 subdivided, but is deep, and rather narrow, usually ap- 

 pearing as an oblique pit in the centre of the crown. There 

 is no enlargement of the incisors to make caniniform teeth. 

 Both the upper incisors and the canine have in the crown 

 a longitudinal groove, which on wear becomes a pit, and 

 being shallow may disappear entirely. The lower teeth 

 are those typical of all toxodonts. The feet are tridactyl, 

 and compact. 



The following three genera may be distinguished : 



RHYNCHIPPUS MORPHIPPUS EURYGENIOPS 



3143 3143 3143 



Formula TTTT TTTT TTTT 



Skull moderately long muzzle short muzzle, with short heavy muzzle, 



slight constriction be- with marked constric- 

 hind canines tion behind canines 



Upper incisors groove or pit groove or pit groove or pit 



