DAKOTA AND NEBRASKA. 333 



teeth, which exhibit differences equal to those of the upper jaw specimens, as seen in 

 figures 17 — 20 of the same plate, representing teeth of four of the fossils of difierent 

 sizes and ages, magnified three diameters. 



The youngest specimen exhibits a full series of five inferior molars, as represented 

 in figure 17. Of these, the anterior four bear a near resemblance in form and consti- 

 tution with the corresponding series of Tltanomys visenoviensis, as represented in plate 

 46 of Gervais' Paleontologie Francaise. 



The first molar forms a double column, with a triturating surface consisting of the 

 ellipses conjoined by a median isthmus. The succeeding pair of molars are the largest 

 of the series, and each is composed of a pair of columns, which on the triturating sur- 

 face are only united by cement. The anterior column is the broader, and trans- 

 versely ellipsoidal ; the posterior column is somewhat pyriform in transverse section. 

 The fourth tooth is about the size of the first, and formed like the second and third. 

 The fifth and last tooth, the smallest of the series, is in excess of the number existing 

 in Tltanomys. In form it resembles that of the Rabbit, and the first one of its own 

 series. 



A second specimen, of more advanced age than the preceding, contains the anterior 

 four molars, as represented in figure 18, together with the alveolus for a fifth tooth. 

 The columns, composing the molars, on the triturating surfaces are seen to be united 

 intei'nally by a narrow isthmus. In addition, the first molar exhibits its median 

 isthmus. 



A third specimen contains the anterior three molars, as represented in figure 19. 

 The teeth are intermediate in appearance to those of the preceding two specimens. 

 On the triturating surfiices the columns of each tooth, except the first, appear united 

 only by cementura. 



A fourth specimen contains the anterior three molars, as represented in figure 20. 

 The jaw fragment is of more robust proportions than any of the other corresponding 

 specimens. The teeth indicate an aged animal, and in general appearance hold a 

 near resemblance with those of an upper jaw fragment, the teeth of which are repre- 

 sented in figure 16. The crowns are very much worn away, and are inserted into 

 the jaw by fangs. The crowns are mutilated internally, but appear to have been 

 devoid of enamel in that position. The triturating surface of the first molar exhibits 

 no transverse valleys, and is bordered by enamel only externally. The succeeding 

 teeth exhibit an external valley, and are bordered by enamel externally and partly 

 at the sides. 



If all the specimens above described really belong to the same species, as is sup- 

 posed, the changes which take place in the teeth in the advance of age appear briefly 

 to be as follow : 



The upper molars are double columns, which are at first separated by an internal 



