1910.] Amphitheriiun, its Place in Mammalian Phylogeny. 179 



(1) Upper molars (in DnjoJesies) very wide transversely, short antero- 

 posteriorly and not really triangular. 



(2) Lower molars (exc'ei)t in Dicrocijnodon) short antero-posteriorly 

 as compared with those of higher mammals, with an asymmetrical trigonid 

 and an incipient protolojjhid. 



Importance of Amphitherium, 



Its primitive characters. The most generalized member of the order and 

 one which probably possessed all the characters listed above is the famous 

 genus A mphitheriuni, which so far as its mandibular and dental characters 

 indicate might re]>resent the common stock of the later Polyprotodont 

 Marsupials, and Placental Insectivores and Creodonts. 



Two great facts about Amphitherium are first, that it is a very primitive 

 Middle Jurassic mammal in which the tuberculo-sectorial type of lower 

 molar is seen in a generalized condition and secondly that from the general 

 similarity of its lower molars to those of Didelphis and many other mammals 

 with tuberculosectorial lower molars we can infer with great probability 

 that the upper molars will also be found to be of a generalized " tritubercular" 

 type. Amphitherium, then, occupies a most important place in mammalian 

 phylogeny in general, and in the Theory of Trituberculy in particular. 



Lower teeth of Amphitherium not derived from the Triconodont type. 

 The cheek teeth of Amphitherium prevostii Blainville as described very 

 carefully by Goodrich (1894, pp. 414-415), offer so good a comparison with 

 those of Didelphis that Cuvier described the jaw of Amphitherium as "celle 

 d'un petit carnassier dont les machelieres resemblent beaucoup a celles des 

 Sarigues, mais il y a dix dents en serie, nombre que ne montre aucun car- 

 nassier connu" (quoted by Goodrich, /. c, p. 410). They are more primitive 

 than those of Didelphis in several respects: 



(1) The number of true molars is greater (as six is to four) and the 

 individual teeth are relatively smaller; there appear also to be five premolars 

 (Goodrich, Osborn). 



(2) The protoconids are higher, and the para-, meta- and entoconids 

 lower, the talonids much smaller; the latter fact implies a high protocone 

 (pp. 175, 180). 



Amphitherium. agrees with Didelphis, with the other Trituberculates and 

 with primitive mammals generally in the fact that the trigonid is not sym- 

 metrical, the metaconid being almost directly internal to the protoconid. 

 This important fact seems to have been neglected hitherto in the develop- 

 ment of. the theory of Trituberculy (p. 187). The paraconid, metaconid and 

 entoconid are also in the same antero-posterior line and when compared 



