160 



EVOLUTION OF MAMMALIAN MOLAR TEETH 



a process the reverse of concrescence. He concludes (p. 379), that 

 as each tooth has primitively sprung by concrescence of cusps derived 

 from successive dentitions, so in incipient retrogression these cusps 

 break apart again into their original components. 



A 



m.2 m.3 



FIG. 132. Evolution of the upper molars in the Notharctidae, a family of American Eocene 

 Primates. A, Peli/iMlusj'sviiiroi-tis, Wasatch Formation (Lower Eocene), the teeth showing clear 

 traces of trituberculy. B, Notharctus nunienus, Wasatch Formation, p* and nii-m3 more quadrate. 

 C, Notharctus sp., Bridger Formation (Middle Eocene), p* quadrate, with two external cusps; 

 vnl-m ;5 large, with well-developed mesostyle (ms). x . 



p.3 p. 4 m.l m.2 



m.3 



FIG. 133. Notharctus sp., Bridger Formation, Middle Eocene. Crown view, lower teeth, 

 showing the loss of the paraconid in ni^m*, enlargement of the postero-internal cusp or 

 entoconid. x ^. 



Apart from this new evidence for the concrescence theory, to 

 which we do not feel able to attach much weight, the question of the 

 existence of the progressive evolution of the ' tuberculus anomalus ' or 

 protostyle in the human teeth is especially interesting, as another 

 instance of homoplasy , or the independent evolution of an apparently 

 homologous cusp in different orders (see p. 236). 



