90 THE DESEADO FORMATION OF PATAGNOIA 



RHYNCHIPPUS MORPHIPPUS EURYGENIOPS 



Lower incisors cingulum on the inner no cingulum no cingulum 



face 



Upper premolars cingulum on ant. int. cingulum on ant. int. cingulum on ant. int. 

 corner corner corner 



Upper molars basin deep basin shallow basin deep, with incipi- 



ent secondary bays 



Lower molars 4 bays bays i and 2 only bays i and 2 only 



Rhynchippus Ameghino 



Rhynchippus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 462. 



The teeth of both jaws are rooted, but tend to be hypso- 

 dont. The elongated incisors of the upper series are char- 

 acterized by the presence of a longitudinal furrow in the 

 top of each tooth, which, with wear, becomes a pit, and, 

 as it is shallow, disappears in old individuals. This is the 

 only suggestion of a horse character in the genus, but the 

 pit in a horse's incisor is a late development, and here it is 

 also probably a specialization due to eating grass. Incisor 

 I is the largest and they decrease in size toward either side. 

 The canine is small, and is also marked by having a furrow 

 in the crown, but in this case it is transverse to the long 

 axis of the jaw. 



The premolars are peculiar in having on the anterior 

 internal corner a highly developed cingulum, which so 

 builds out the tooth that it is usually wide and is rectangu- 

 lar in outline. As this cingulum rises, it incloses a bay on 

 the ant. int. corner of the tooth, which, with wear, becomes 

 first a bay, then a pit, and lastly may disappear entirely 

 in old age. On each premolar the anterior and posterior 

 lobes are developed, inclosing between them an elongated 

 basin, which with wear becomes a long narrow pit. On 

 the molars, the cingulum on the ant. int. corner is wanting 

 entirely. The external anterior corner of the tooth, how- 

 ever, is prolonged, so that the crown has a rhomboidal out- 

 line. The crown is made up in the typical manner of the 

 wall, the anterior, and the posterior lobes, which inclose 



