rele) 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 1 and 2 only bays T 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 
