SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 9 



one incisor has been lost from each premaxilla, and that the tusk is not i- 

 is shown by two considerations: (i) In some individuals of TJioatheriiim 

 a very small deciduous incisor is inserted in the premaxilla behind the tem- 

 porary tusk; (2) the fact that the lower tusk bites behind i\\& upper is evi- 

 dence that the former is one degree farther back and therefore represents is. 



The upper canine is always absent and the lower, though present in all 

 cases, is much reduced in size and can have had no functional significance. 



The premolars are never altogether molariform, though p- approximates 

 the molar pattern, and p4 and rarely ps have that pattern quite completely. 

 Though characteristically differing in the various genera, the upper molars 

 are much alike throughout the family and have two well-defined, external, 

 concave cusps, with two conical internal cusps and anterior and posterior 

 conules. The transverse crests, especially the posterior one, are incom- 

 pletely formed by the fusion of the internal cusps and the conules, and there 

 is a tendency for all of these internal elements to become confluent and 

 form an antero-posterior ridge. The varying extent to which this con- 

 fluence is carried, forms one of the diagnostic features of the different 

 genera. Where the tendency is least pronounced, as in Diadiaphoms, 

 there is a certain resemblance to the molar-pattern of the Palseotheriidae, 

 though differing in the absence of the posterior transverse crest, or 

 metaloph. 



The lower molars have the bicrescentic pattern so common in early and 

 primitive families of both Artiodactyla and Perissodactyla, such as the 

 Anoplotheriidse, Palasotheriidae, Titanotheriidse, etc., but often with the 

 addition of the characteristic pillar in the concavity of the posterior crescent. 

 This element, so general among the South American ungulates, is found 

 in the Santa Cruz Proterotheriidse in varying stages of reduction ; it may 

 be quite conspicuous [Licaphrhmi), very minute and observable only in 

 perfectly unworn teeth [Proterotherhmi), or tnWvt\yvj2ini\ng[Diadiaphorzis, 

 Thoatherium). When present, this element is an isolated conical pillar, not 

 connected with the outer wall by a transverse spur or ridge, as it typically 

 is in the Macrauchenidas. The last lower molar may either be without 

 any talon ( Thoatherium, Diadiaphorus) or may have a very small one 

 i^Proterotheriuni), or a larger one [Licaphrium). 



The skull has a moderately capacious and well rounded brain-case, with 

 high and thin sagittal crest and orbits completely encircled with bone. The 

 tympanic was so loosely attached that it has been lost from all the avail- 



