VIL] THE EXTERNAL SKELETON, 267 



of the front, inner cusp of man the hind inner one having 

 disappeared. 



In the true molars of the Dog this last cusp reappears, and 

 in addition we find a large prominence of the internal part 

 of the cingulum. In the Badger this latter structure is very 

 much developed, extending in a marked manner along the 





FIG. 241. GRINDING SURFACE OF FIRST RIGHT UPPER MOLAR TOOTH OF 

 A Hums fulgens. 



i, 2, 3, and 4, cusps of the external cingulum ; ic, internal cingulum ; a, b, c, and 

 d, the four principal cusps. 



whole inner margin of the tooth. In Ailurus we have a 

 good example of the maximum of complexity of the carni- 

 vorous type of molar. We find in the molar of this 

 animal four principal cusps, with three cusps belonging to 

 the external cingulum, and two belonging to the internal 

 cingulum. 



Returning now to the type of structure exhibited by man's 

 upper molar, we may follow through another series of ani- 

 mals yet a new chain of modifications, resulting in a very 

 different kind of complication. 



First, one transverse ridge may connect the two posterior 



MACROxCELIDES 

 ft 



FIG. 242. GRINDING SURFACE OF LEFT UPPER MOLAR, showing the transverse 

 ridges (convex forwards) which connect together the anterior and posterior 

 pairs of principal cusps respectively ; a, b, c, and d, the four principal cusps. 



cusps, and another may similarly unite the two anterior cusps. 

 We see this in the Kangaroo and Macroscelides. 



Next, a ridge may run along the external margin of the 

 tooth (probably the cingulum) and connect together the two 



