OUR FACE FROM FISH TO MAN 



to man in the reckless manner of some of the 

 older comparative anatomists, but that the same 

 general process of tooth development may be 

 traced in many successive grades in the ascent 

 from fish to man. 



Meanwhile (Fig. 7lC) Meckel's cartilage, the 

 descendant of the primary lower jaw of the shark, 

 lies entirely free from the future dentary or lower 

 jaw bone, which will later surround both the 

 Meckel's cartilage and the developing tooth-germ, 

 as in all the vertebrates above the shark. 



In the earlier creatures that lie in or near the 

 line of ascent to man the teeth were of the dog- 

 tooth or canine type (Fig. 50) . Some of the front 

 teeth of man, especially the cuspids or canines, 

 remain single-cusped to this day as souvenirs of 

 our remote carnivorous ancestors; but the central 

 incisors often exhibit a tendency to develop little 

 cusps, mammillae or subdivisions, along the flat- 

 tened cutting edge of the crown (Fig. 72). The 

 frequent presence of these mammillae on the edges 

 of the central incisors has sometimes been cited as 

 evidence of a " triconodont " stage in the evolution 

 of human teeth, in disregard of the fact that not 



even the extinct triconodont mammals of the 



136 



