I I !2 DR C. W. ANDREWS ON THE 



I I a ns\ erselv, there is also a small inner cone behind (lie chief one. The cingultUXl is 

 fairly well developed and forms a small prominence both in front and behind. The 



last premolar (I'm. 4) is bilophodont, each ridge consisting of a pair of tubercles. The 

 postero internal cusp is continued backward into a small talon-like portion of the 

 cingulum. 



The first true molar (M. I) is trilophodont, the inner cusp of each ridge is the larger 

 and most worn, the outer is the higher ; the internal ensps tends to unite longitudinally 

 in wear. Tin- second molar (M. 2) is mnch larger than M. I, the sudden increase in 

 size being most striking; its structure is similar to that of the tooth in front; the 

 postero-intemal cusp is united to the talon-like cingulum. The posterior molar (M. 3) 

 is similar in most respects to M. 2, but the hind lobe is much smaller, so that the 

 tooth is actually simpler than thai in front of it. 



The Lmn /• Dentition. The single pair of lower incisors are in close contact along 

 their straight inner borders, and their concave upper surface forms an anterior 



prolongation of the s] t-like symphysis. The upper surface of their anterior and 



antero-external borders is considerably worn. 



The anterior premolar (Pm. 3) consists of a st rongly compressed main cusp with 

 small accessory cusps in front and behind. The last premolar (Pm. 4) is a narrow 

 bilophodont tooth, each ridge consisting of a pair of cusps. There are also small 

 accessory cusps on the anterior and posterior holders of the tooth. The first molar is 

 a small trilophodont tooth with a -small talon. The second molar (M. 2) is much larger, 

 but is likew ise trilophodont ; there is a small anterior cusp connected with the 

 antero-external main cusp, and also a small posterior talon connected with the postero- 

 external main cusp. The posterior molar is similar but the talon is considerably 

 larger. 



Pahmmastodon is more primitive than Tetrabelodon in the following points. The 

 basis cranii and the facial region of the maxilla are longer ; the zygomatic arch is 

 larger and the jugal not only takes a greater share in its composition but also extends 

 farther forw ard. In the mandible the symphysis is not so extremely elongated, and the 

 lower incisors are more normal and less tusk-like. It is in the cheek-teeth, however, 

 that the more generalised character of Pal ceomastodon is most manifest. Thus the 

 molars and premolars are all in wear at the same time and their mode of succession is 

 almost as in ordinary Ungulates, such as the pig. The premolars replace milk 

 predecessors, and the molars come into use one after the other in the usual way, the 

 first molar being always the most worn. As to the structure of the individual teeth, 

 it may be said that they are still more brachydont than in Tetrabelodon, the main 

 tubercles of the transverse ridges more distinct from one another and the accessory 

 tubercles less developed. 



In both genera the first and second molars are trilophodont, but in Tetrabelodon 

 the last molar is much the more complex in both upper and lower jaws (see figs. 7 

 and 9). The sudden increase in the size of the last two molars in PaUeomastodon seems 



