SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 57 



In ProterotJiermni the premaxillse differ from those of DiadiapJiov2ts in 

 the much smaller part which is broad and thick, with rounded and de- 

 pressed dorsal surface, and in the far broader ascending ramus, which has 

 a thinner and much more nearly vertical anterior border ; the incisive 

 foramina are relatively larger, but are entirely confined to the premaxillse. 

 Owing to the differences of form of the nasals and premaxillaries, the 

 anterior nares have quite a different shape in the two genera. The max- 

 illaries are very similar in both, but in ProterotJieriiim the infraorbital 

 foramen is proportionally much larger, though still very small, single and 

 placed nearer to the orbit. The palatines are very long, extending for- 

 ward to m- or p-, and the hard palate is narrow and concave, while the 

 posterior nares are not produced so far forward as in DiadiapJiorus. 



The mandible differs but little from that of the last named genus ; the 

 symphysis is broader and more depressed, less concave on the dorsal side 

 and more procumbent, the ventral border rising very gradually to the 

 incisive alveolus. The coronoid, which is high and narrow, has a greater 

 inclination forward, especially in young animals, and the condyle is nar- 

 rower antero-posteriorly, more convex transversely. 



Skeleton. — Except for smaller size and more slender proportions, all 

 the bones of the skeleton closely resemble those of Diadiaphoriis. Very 

 few vertebrae have as yet been found in association with any of the speci- 

 mens. The atlas has a low, but distinct neural spine, and a well defined 

 hypapophysis, which projects backward from the hinder border of the 

 inferior arch: the atlanteo-diapophysial notch is in process of conversion 

 into a foramen, as is indicated by a short, pointed process arising from 

 the anterior border of the notch ; the depression for the odontoid process 

 of the axis is narrow and deeply concave. 



The lumbar vertebrae, so far as they are known, resemble those of Dia- 

 diaphorus in structure, though the metapophyses are relatively more 

 prominent. On the last lumbar the transverse processes are extremely 

 broad and articulate with those of the first sacral by deep concavities, as 

 appears to be the case in all of the Litopterna. The number of sacral 

 vertebrae is not definitely known, but probably exceeded six. The first 

 vertebra (PL XII, fig. 2) is very broad, but behind this the sacrum rapidly 

 contracts, becoming very narrow and very shallow dorso-ventrally, which 

 indicates that the tail was short : the coalesced neural spines form a low 

 ridge, with a somewhat higher ridge on each side of it, apparently due to 

 the fused metapophyses. 



