SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 85 



first upper milk-premolar (dp-) is larger than p- ; its crown is made up of 

 a very large external cordate cusp and two internal cusps, of which the 

 anterior is very small and hardly more than a raised cingulum ; on the 

 outer side of the tooth, near the anterior border, is a deep vertical groove. 

 The second upper temporary premolar is of similar pattern, but much 

 larger and with better developed internal cusps. The third is like a 

 molar, with two external cusps, but has a relatively greater antero-pos- 

 terior elongation, while dp- is altogether molariform. 



The difference in size between the median and lateral incisors of the 

 mandible is even less in the temporary than in the permanent dentition 

 and the milk-canine is smaller than its successor. In the premolar series, 

 dpi is like pr, but dp2 is larger than p2 and has quite a different appear- 

 ance ; of the two internal cusps, the anterior one is better developed and 

 the posterior one less so, while the ridge that runs back from the summit 

 of the protoconid ceases at the postero-internal cusp, while in p2 it is con- 

 tinued for the whole length of the crown. The last two milk-premolars 

 differ from p-g- and 4 only in their greater elongation antero-posteriorly. 



Skull (Pis. XIII, fig. 12 ; XIV). — In general, the skull resembles that 

 of Proterotherium, but is smaller, lighter, more slender and graceful, and 

 there are several constant differences in details. In the first place, there 

 are certain differences of proportion, which though not striking, are yet 

 not without importance. The cranium is somewhat shorter and the face 

 correspondingly longer, the orbit, which is relatively larger, having a 

 rather more posterior position : the sagittal crest is considerably shorter 

 and the temporal foramina have a conspicuously shorter antero-posterior 

 diameter ; the face tapers forward more gradually and the muzzle is 

 narrower and more slender. Another and very characteristic difference is 

 in the shape of the occiput, which is quite as broad at the base as in 

 Proterotherium, but narrows rapidly, making the dorsal moiety extremely 

 narrow and pointed, like a slender, lanceolate arch : this dorsal portion of 

 the occiput projects backward much farther than in Proterotherium. The 

 basioccipital is relatively broader than in the latter and has a better defined 

 median keel and the occipital condyles are more widely separated ventrally. 



The brain-case is shorter, fuller and more rounded than in Protero- 

 therium and descends more abruptly behind, making the posterior part 

 of the sagittal crest very much higher. The squamosal is much as in the 

 latter genus, but the postglenoid process is shorter and has a more 



