190 EVOLUTION OF MAMMALIAN MOLAR TEETH 



Protcrothcrium (Fig. 192), for example, we see the trigonal disposition 

 of three main cusps. The still more ancient Ungulates from the 

 Notostylops beds of Patagonia, exhibit molar teeth of the type seen 

 among the Amblypoda and Condylarthra, namely the tritubercular, 

 bunodont type, and lend the strongest of all the recent evidence which 

 has come forward in support of the tritubercular theory. 



Pyrotherium, believed by Ameghino to be ancestral to the Pro- 

 boscidea, has simple, bilophodont molars. 



Doctor Ameghino, although rejecting Osborn's homologies of the 

 molar cusps, and holding widely divergent views as to the ultimate 

 irigin of the molars, yet brings forward a great deal of evidence l to 

 show the derivation of all the inferior molar types of South American 

 orders from a Proteodidelphys (Fig. 202) type which has a typically 

 tuberculosectorial lower dentition more primitive even than that of 

 Diilclpliys (see pp. 202, 204). 



SPECIAL REFERENCES. 



Ameghino, F., Contribution cd Conocimiento de los Mamniiferos Fosiles de la 

 Republica Argentina. Buenos Aiies, 1889. Numerous contributions to Anales del 

 Museo National de Buenos Aires, Boletin del Instituto Geograjico Argentina, Boletin 

 Acad. Nac. Ciencias Cordoba, Anales Soc. Cientifica, Argentina, etc. ; especially 

 " Recherches de Morphologie Phylogunetique sur les Molaires Superieures des 

 Ongules," An. d. Mus. Nac. des Buenos Aires, Tom. IX., 1904. 



Lydekker, R., Palaeontologia Argentina II. La Plata, 1893. 



Owen, R., The Zoology of H.M.S. Beagle, Pt. L, "Fossil Mammalia," London, 1840. 



CETACEA. 



Aquatic adaptation has gone to such an extreme in the teeth 

 of the Cetacea that all traces of tritubercular ancestry, if such ever 

 existed, have been entirely obliterated. It has been suggested that 

 the Cetacea are so ancient that they branched oft' before the 

 haplodont reptilian crown had begun the series of modifications 

 leading to trituberculy (Fig. 43), but the presence of accessory cuspules 

 in the posterior molars of certain recent and Miocene Platanistidse 

 and of vestigial low cusped, two rooted teeth in embryos of Whale- 

 bone Whales, the analogy with the secondary haplodont molar teeth 

 of certain Pinnipedia (Fig. 103), and the fact that the placenta tion 

 and reproductive organs of the Cetacea are of a very high Eutheriaii 

 type, are all more in accordance with the hypothesis that the ancestors 

 of the Cetacea possessed more complicated tooth crowns, and that the 

 existing haplodont types are all secondary. 



1 " On the Primitive Type of the Plexodont Molars of Mammals," Proc. Zoo!. Soc. 

 Lond., May -2, 1899, pp. 555-571. 



