212 ON THE EXTINCT MAMMALIA OF 



of the crown externally. Between the points mentioned the lobes embrace concave 

 valleys opening in an angular manner internally. 



As the lower true molars were worn (see figure 3, plate xvi, figures 8, 9, plate 

 xvii, Anc. Fauna of Neb.), crescentic tracts of dentine became exposed upon the 

 abraded summits of the lobes. These tracts gradually widened, and those of the 

 contiguous lobes became conjoined. The abrasion continuing, the internal concave 

 valleys of the lobes became gradually obliterated, leavmg upon the crown a broad 

 yoke-shaped tract of dentine bordered by thick enamel. 



Premolars. — A fragment of a lower jaw belonging to the collection of Prof Hall 

 indicates the existence of four premolars, but none of the teeth have been preserved 

 in the specimen. 



An isolated, much worn premolar, apparently a third, is represented in figures 8 — 

 10, plate xvi, of the Ancient Fauna of Nebraska. It has a bi-lobed crown of the 

 same form as in the true molars, but is much smaller. The worn triturating surface 

 exhibits a broad yoke-like tract of dentine bordered by enamel. 



Figures 9 — 12, plate XXIV, represent two isolated premolars, probably the second 

 and first. They have likewise bi-lobed crowns constructed after the pattern of those 

 of the true molars, but much reduced in size, and m a more rudimental condition of 

 development. 



Figure 8 represents the crown of an isolated superior premolar, probably the 

 second of the temporary series. The anterior extremity is worn off apparently by 

 the lateral pressure of a first lower premolar. The specimen differs from the perma- 

 nent premolars not only in the prolongation of its fore part, but also in the greater 

 distinction of the constituent lobes, the inner ones being separated as much as in the 

 upper permanent true molars. The tooth may probably belong to an animal quite 

 distinct from Titanothermm. 



Canines. — The canine teeth of Titanotlierium are of moderate proportions, and are 

 situated but a short distance in advance of the molar series. 



In the upper jaw specimen, figs. 1, 2, plate XXIV, of Prof. Hall's collection, the 

 sockets for the canines are curved conical, and measure at their exit rather more 

 than an inch in diameter. 



The figures 11, 12, of plate xvi, of the Ancient Fauna of Nebraska, represent the 

 crown of a canine tooth supposed to have belonged to the lower jaw represented in 

 figure 2 of the same plate. It is curved conical, with the inner smaller and less con- 

 vex surface defined from the outer by rather obtuse or sub-acute ridges. It is pro- 

 vided with a basal ridge, which is more strongly developed internally. The apex of 

 the crowTi is worn ofi", and the wearing extends downward a short distance externally 

 so as to exhibit there an oval tract of exposed dentine. The length of the crown is 

 over an inch and a half; its diameter from before backward at the base an inch. 



