134 



CHORDATE ANATOMY 



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 con- 

 crescence theory seems to have less factual support than does the differen- 

 tiation 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. 



A B C D E 



Fig. 124. — A, triconodont tooth of Dromatherium; 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) ; I, talon. 

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



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

 generations of teeth Hke the milk and permanent sets, that is, their denti- 

 tion was polyphyodont. Under these conditions, the germs of successive 

 sets might fuse with one another. The factual foundations of the theory, 

 however, are weak. 



TEETH OF MAMMALS 



Teeth of mammals are especially important for the paleontologist, 

 partly because they are hard and therefore Hkely to be preserved, but 

 more because mammalian teeth are closely correlated with feeding habits. 

 But feeding habits, in their turn, are correlated with the entire bodily 

 structure, so that teeth are a key to the whole organism. Moreover, 

 mammalian teeth are so highly specialized and so diverse in size and 

 structure, that a single one is often sufficient to identify a species. 



In general, the tendency has been to reduce the number and to do 

 away with the division into two sets, and at the same time to specialize 

 and elaborate individual teeth. An ideally complete set for a placental 



