PETERSON: THE AMERICAN DICERATHERES. 453 



of considerable size. The two succeeding round openings are for the roots of 

 D.P2. Back of this point the two succeeding cheek-teeth are partly erupted. 

 (See PI. LXVI, Figs. 3, 8 and 9.) The general pattern of these teeth is quite 

 similar to that of the permanent set, indeed it is not easy to distinguish one set 

 from the other. Portions of a fifth cheek-tooth (Mi) lie buried deep in the jaw 

 behind the two just described. 



4. Two pairs of lower jaws (Nos. 2476, 2477) have been selected from the 

 collection to represent the next stage of evolution in the development of the ramus. 

 No. 2476 being represented on PL LXVI, Fig. 7. The total length of jaws Nos. 

 2476, 2477 is 250 and 270 mm. and their depth is 36 and 40 mm. respectively. At 

 this stage the jaw is easily recognizable, as all the characteristic generic features 

 are present. The jaw is less lunate-shaped, the characteristic diastema in front 

 of the cheek-teeth is quite well developed, including the constriction of the alveolar 

 border, which in the younger stage is represented only by the deep groove on the 

 external face of the ramus referred to above. In this stage of development the chin 

 is broader, due to the lodgment of the already well-advanced lateral incisor. The 

 temporal fossa is well developed, as are the condyle and the coronoid process. 



The median incisors are just through the alveolar border and present the 

 same small and conical-shaped crowns met with in older forms. These teeth 

 are succeeded by a short diastema before the alveoli of D.I 2 is reached. The latter 

 is situated somewhat posterior to Ii and I3 in the alveolar border and is thus placed 

 in an irregular position. In the .specimens of the Carnegie Museum under obser- 

 vation there is sometimes found a delicate septum separating the second and third 

 incisors. This bony bridge is often broken. D.I3 is frequently found in i:)osition 

 as is the case in the specimen here described, see PI. LXVI, Fig. 7. This tooth has 

 a long root, quite robust, on which sits a small enamel-covered crown very little 

 larger in circumference than the root itself. The two lateral incisors are succeeded 

 by a diastema; the alveolar border here forms a heavy rounded edge with a notice- 

 able swelling on the external face. This swelling of the incisive alveolar border 

 is due entirely to the rapid development of the cutting incisor of the second set of 

 teeth which is yet buried underneath the deciduous dentition. The deciduous 

 canine is found in many individuals. This is a small tooth with a conical enamel- 

 covered crown and a rather short root. The canine is quite generally found in a 

 procumbent position, isolated by diastemata in front and behind, and drops out 

 very early. The diastema back of the canine constitute a long and sharp border 

 which has first a slight inward curvature and then suddenly takes an outward 

 direction to meet the cheek dentition. The first deciduous cheek-tooth is rather 



