DAKOTA AND NEBRASKA. 229 



small internal incisor. The mental foramen is situated beneath the back half of the 

 molar tooth. 



The crown of the upper deciduous molar, represented in figures 4, 5, appears to 

 belong to the third or fourth of the series of the right side. It is moderately worn, a 

 continuous tract of dentine being exposed along the summits of all the lobes. 



The outer part of the crown forms a thick wall, with the inner surface sloping 

 inwardly, and with the outer surface convex downward and divided near its middle 

 by an obtuse ridge. From the outer wall of the crown there extend inwardly a pair 

 of transverse lobes, embraced at bottom by a well-developed and deep basal ridge 

 extending from one extremity of the outer wall of the crown to the other. The 

 internal lobes expand inwardly and towards their base. The anterior of the two 

 lobes curves backward in its course inward. A deep valley separates the internal 

 lobes, and shallower ones bound them in front and behind. From the outer wall, 

 two narrow folds or processes project into the wide outer extremity of the middle 

 valley. One of the processes joins a thicker process or offset projecting at right angles 

 from the posterior transverse lobe, and thus contributes to convert a portion of the 

 middle valley into a deep pit. The anterior and posterior valleys, of which the latter 

 is the deeper and longer, are bounded opposite the lobes by the basal ridge. 



The fore-and-aft diameter of the si^ecimen is nineteen lines; its transverse diameter 

 anteriorly sixteen lines, posteriorly nineteen lines. 



Rhikoceros meridianus. 



The species to which the above name has been applied is indicated by the greater 

 and more characteristic portion of the crown of an upper molar tooth, submitted to 

 the examination of the writer by Dr. Benjamin F. Shumard, of St. Louis. It was 

 derived from a tertiary deposit in Washington county, Texas, and presents much the 

 general aspect of the Mauvaises Terres fossils of White River, Dakota, with which 

 it is j^robably of cotemporary age. 



The specimen is represented in figure 10, plate XXIII, and appears to belong to 

 the penultimate molar of the right side. It evidently indicates a species different 

 from R. occidentalis and B, crassvs, and was intermediate in size to them. 



The estimated measurements of the tooth in a restored condition are two inches 

 fore, and aft externally, twenty-two lines wide in front, and eighteen lines wide 

 behind. The median valley of the crown is strongly sigmoid in its course, arising 

 from the inner lobes being each provided with an oblique offset alternating and ex- 

 tending into the valley in a parallel manner, as seen in the figure. 



