COMPARATIVE ANATOMY 



the lower jaw are called protoconid, paraconid and metaconid. Teeth of 

 this sort are known as triconodont. Besides the three cones, triconodont 

 teeth have a basal rim, the cingulmn, which forms part of the crown. 

 Marsupial-like mammals of the Tertiary had teeth of this triconodont 

 sort. See Fig. 143. 



The secondary tubercles of such teeth show a tendency to enlarge 

 to the size of the protocone. A further advance occurs when the three 

 cones assume a triangular relation to one another, the secondary cones 

 of the upper jaw migrating inwards, those of the lower jaw outwards. 

 Teeth of this tritubercular sort occur in Amphitherium of the Jurassic 

 period. 



Later, in mammals, appeared a posterior projection or talon, and a 

 fourth tubercle, the hypocone and hypoconid. With these additions, 

 the molar teeth assumed more and more the modern form with six cusps. 



Fig. 143. — A, triconodont tooth of Dromalheriiun; B, tritubercular tooth of Spalaco- 

 therium; C, interlocking of upper (dark) and lower (light) tritubercular molar teeth 

 (after Osborn) ; D, molar of Erinaceus; E, of horse (selenodont type); c, cingulum; m, 

 metacone (metaconid) ; pa, paracone (paraconid) ; pr, protocone (protoconid) ; t, talon. 

 (From Kingsley's "Comparative Anatomy of Vertebrates.") 



It took many million years to accomplish these changes, which were 

 naturally based upon change in the form of the tooth-germ and involved 

 budding of that organ. 



The concrescence theory accounts for the multitubercular molar 

 teeth of mammals by supposing a fusion of the anlagen of conical teeth, 

 the number of cusps corresponding with the number of conical teeth 

 involved. Some observers claim to have found evidence of fusion of 

 tooth-germs in vertebrate embryos, but most investigators are sceptical. 

 It must be said, however, that tooth fusion is known to occur in the case 

 of the massive pavement teeth of dipnoi. At the present time the 

 concrescence theory seems to have less factual support than does the 

 differentiation theory. 



According to Bolk, in a modified form of the concrescence theory, 

 compound teeth are formed by the fusion of the germs of successive sets. 

 His theory assumes that the ancestors of mammals had more than two 

 generations of teeth like the milk and permanent sets, that is, their 

 dentition was polyphyodont. Under these conditions, the germs of 



