RHYNCHIPPUS 91 



between them an elongated basin, which, as in the pre- 

 molars, becomes, on wear, a pit extending obliquely across 

 the tooth. 



The lower incisors have no furrows in the crowns, but 

 in this genus there is a small cingulum on the inner side 

 just above the base of the enamel. The lower canine is 

 incisiform, and also has the basal cingulum. Each of the 

 premolars has, on the external side, a median vertical 

 groove, beginning at the base of the enamel, and widening 

 toward the top. This is progressive if less marked from 

 pm. i to pm. 4. The premolars and molars consist essen- 

 tially of two crescents, the shorter anterior, and the pos- 

 terior which is about twice as long as the anterior. The 

 details are as described on page 96, and seen in figure 55. 



The skull is of moderate height, nearly flat on top with 

 wide zygomatic arches. The sagittal crest is moderately 

 high, and slightly convex in the antero-posterior direction. 

 The occipital region is overhanging and topped by short 

 lambdoidal crests, which, extending to either side, unite 

 with the zygomatic arches. The nasals are large, roughly 

 rectangular, and slightly constricted just in front of the 

 middle. The f rentals are short, and project over the orbits 

 in strong processes. The maxilla is large, bounding the 

 front of the orbit, and extending backward in a strong 

 zygomatic process which makes nearly half of the arch. 

 The jugal, while stout, is short, and reaches from the long 

 zygomatic process of the maxilla to the short one of the 

 squamosum. The lachrymal bone is tiny, with a low tu- 

 bercle, just below which is situated the lachrymal duct, 

 just on the margin of the orbit. Just behind the zygomatic 

 arch, the squamosum is inflated and contains a large hol- 

 low chamber, as is typical among toxodonts. The mastoid 

 bullae, while relatively small, are swollen into a globular 

 form, and have a large hollow chamber. ' The palate ex- 

 tends back to just behind the last molar, a feature distin- 

 guishing this genus from Morphippus. 



