332 



ANATOMY OF VERTEBRATES. 



size, with its sectorial form; its blade is divided into two cones by 

 a wide notch, the anterior cone being the strongest and most 

 produced ; the tubercle is developed from the inner side of the 

 base of this lobe. The first and second upper molars, m 1 and 2, 

 are tuberculate ; but the second is very small, less than half the 

 size of the first molar. The first true molar below, m 1, is modi- 

 fied to form the opposing blade to the sectorial tooth above ; re- 

 taining the tuberculate character at its posterior half. The blade 

 is divided by a vertical linear fissure into two cones, behind which 

 the base of the crown extends into a broad trituberculate talon. 

 The second molar, m 2, has two anterior cusps on the same trans- 

 verse line, and a posterior broad flat talon ; the last lower molar, 

 m 3, is the smallest of all the teeth. 



The absence of a tuberculate molar in the lower jaw of the 

 immature Dog, brings the character of the deciduous dentition 

 of the genus Canis, fig. 262, closer to the permanent dentition of 

 stricter carnivores, and affords an interesting illustration of the 



262 



Deciduous and permanent teeth in the Dog {Canis). 



law that unity of organisation is manifested directly as the 

 proximity of the animal to the commencement of its development. 

 The succession of two tubercular molar teeth behind the perma- 

 nent sectorial tooth in the permanent dentition of the lower jaw 

 contributes to adapt the Dog for a greater variety of climates, of 

 food, and of other circumstances, all of which tend, in an important 



