MORPHIPPUS 105 



Rhynchippus medianus Ameghino 



R. medianus Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 375. 



This third species is intermediate in size between the 

 two foregoing. No figure is given, but the following meas- 

 urements give the size: 



Upper molar 2, length, 1 7 mm., width n mm. 



Length of lower molars I to 3 40 mm. 



Height of mandible under m. 2 24 mm. 



These figures would indicate a form about 15 % larger 

 than R. pumulis, and 35 % smaller than R. equinus. 



Morphippus Ameghino 



Morphippus, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 459. 



This genus is very similar to Rhynchippus, with the same 

 dental formula, the same grooves in the upper incisors, and 

 the same pattern of the premolars and molars. It differs, 

 however, in the lower incisors having no cingulum at their 

 base, in the upper molars having a shallower basin, and in 

 bays 3 and 4 being absent from the lower molars. These 

 features simply indicate a slightly less advanced specializa- 

 tion, less hypsodont teeth. I do not think that the bays 

 are any of them lacking in unworn teeth, but in a less hypso- 

 dont tooth, with the pits extending a less distance into the 

 crown, all indication of the bays disappears early. 



M . imbricatus is described as the type species, and four 

 others have been described, all equal in size to M. imbrica- 

 tus, and distinguished by the teeth being slightly more 

 compressed, by the external cleft of the lower molars being 

 deeper, or by variations in the pits. All these features I 

 consider to be either age characters or individual variations, 

 so that all five species of this genus are lumped under M. 

 imbricatus. 



