This family name is used for the three genera _Rhynchippus, 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 associated 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 complete skeleton of _Rhynchippus equinus_ which in all particulars 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 appearing 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 3 1 4 3 3 1 4 3 3 1 4 3 Formula ------- ------- ------- 3 1 4 3 3 1 4 3 3 1 4 3 Skull moderately long short muzzle, short heavy muzzle with slight muzzle, constriction with marked behind canines constriction behind canines Upper incisors groove or pit groove or pit groove or pit Lower incisors cingulum on the no cingulum no cingulum inner face Upper premolars cingulum on ant. int. cingulum int. cingulum on corner on ant. corner ant. int. corner Upper molars basin deep basin shallow basin deep, with incipient secondary bays Lower molars 4 bays bays 1 and 2 bays 1 and 2 only only =Rhynchippus= Ameghino Rhynchippus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 462.