340 



ANATOMY OF VERTEBKATES. 



feeding animal, allied to the viverrine and canine families, the 

 true molars presenting the tuberculate modification, and the 

 typical number and kinds of teeth being functionally developed, 

 as in the Hycenodon. The series in the upper jaw are shown in 

 fig. 267. The term 'tubercular' is as applicable to the three 

 true molars of the Amphicyon, m l, 2, 3, as the term ( carnassial ' 

 is to those of the Hycenodon. 



267 



Dentition, upper jaw, Amphicyon. 



§ 221. Teeth of Ungulata. — The most common characteristic of 

 this dentition is the large size, cuboid shape, and complex structure 

 of the crowns of the grinding teeth. The enamel not only incloses 

 but dips or penetrates into the substance of the dentinal body, 

 and the cement, which is thick, accompanies the enamel. Thus 

 the massive grinding organ is made up of substances of different 

 densities, and the working surface is irregular by the projections 

 of the harder material, as in the mineral ' grit ' that is thereby 

 suitable as a millstone. 



A. Homologies of the parts of the grinding surface. — The pattern 

 of the grinding surface, especially of the upper molars, varies in 

 each genus of Ungulata, and is eminently characteristic thereof. 

 Nevertheless, two leading types may be recognised. One, of un- 

 symmetrical character, was early shown in Palceotherium, and is 

 traceable in secondary modifications characteristic of Paloplothe- 

 rium, Hipparion, Equus, Hyrax, and Rhinoceros. A second was 

 as early manifested in Anoplotherium and Dichodon ; it is more 

 symmetrical in pattern, and is traceable, with modifications, in 

 Dicotyles, Sus, Hippopotamus, and Ruminants. Indications of a 

 more generalised type of molar have been obtained from tertiary 



