26 1 ELEMENTARY ANATOMY. [LESS. 



29. The TRUE MOLARS are very rarely more numerous than 

 in man, who has the typical number of the great (Monodel- 

 phous) division of his class. In the Marsupials (Didelphous 

 Mammals), however, there are typically four above and four 

 below. 



They may (even where pre-molars are present) be quite 

 wanting in the upper jaw, while there is but one on each side 

 (and that very small) of the lower jaw, as in the Cats. 



30. The form of the human true molars, both above and 

 below, will readily serve to explain the more general modifi- 

 cations present in man's class. 



The peculiar form of the upper molar namely, four cusps, 

 one at each angle, with the before described oblique con- 



d 



FIG. 235. GRINDING SURFACE OF A LEFT UPPER MOLAR. 

 a, antero-external cusp ; b, antero-internal cusp ; c, postero-external cusp ; d, 



postero-internal cusp. 



The oblique ridge is seen passing from the postero-external cusp to the antero- 

 internal one. 



necting ridge reappears not only in some of the Apes, but in 



other beasts, as e.g. in the Hedgehog and its ally, Gymnura. 



A " band " of dental substance (termed the cinguluni) may 



surround the tooth, and even in man's own order (Primates) 



30REX 



FIG. 236. GRINDING SURFACE OF A LEFT UPPER MOLAR. 



a and b, anterior cusps ; c, postero-external cusp ; i, 2, 3, and 4, the four acces- 

 sory cusps of the external cingulum. 



may develop small accessory cusps which project downwards 

 external to the two outer of the four principal cusps. 



By further development the external cusps of the band may 



