CREODOXTA. 271 



tional permanent teeth of the placental Mammalia must have appeared later 

 in time than the one already found in the implacentals, they must be those 

 later protruded; hence the fourth tooth in the jaw of Tri'isodon must be re- 

 garded as homologous with the fourth premolar of a placental, which is the 

 last of that series to appear. If this be true, the tooth which follows the 

 shed tooth of the marsupials is not the fourth premolar, as supposed by 

 Professor Flower, but the third premolar. This view is coniirmed by the 

 fact that the milk-tooth displaced by the fourth tooth in Triisodon resembles 

 in all respects the true molai's, just as the permanent tooth occupying the 

 same position does in Didelphys and some extinct Eocene genera. This goes 

 to show that this tooth, permanent in marsupials, is temporary in placentals, 

 and that, in spite of its form in the former group, it is the fourth premolar 

 and not the first true molar, as supposed by Professor Flower. Thus the 

 posterior milk-molar of diphyodonts is a permanent tooth in the MarsupiaUa. 



This observation confirms my conclusion that the Creodonta form a 

 group intermediate between the MarsupiaUa and Carnivora} I may add 

 that in Triisodon the inferior border of the lower jaw is not inflected pos- 

 teriorly. 



Four species of this genus are known, which differ in some points of 

 dental structure as well as in size. Their characters are as follows: 



I. Internal anterior cusp of inferior true molars very small and well separated 



from external anterior cusp. 

 Length of inferior true molars .044 ..T. quiviremis. 



II. Internal anterior cusp of inferior true molars larger and nearly connected with 



the external. 

 Heel of inferior true mol-'rs simple; length of true molars .023; smaller . . T. levisanus. 

 Length of inferior second true molar .011; heel with several lobes; larger. 



T. heilprinianus. 

 Length of inferior true molars .052; heel with two tubercles; largest T. conidens. 



The .superior molar teeth show a resemblance to those of Mesonyx and 

 also to those of DeUatheriwm. Among the Mesonychidce, Tri'isodon approaches 

 Sarcothraustes in the form of the inferior molars, in the expanded heel. On 

 the other hand, the appearance of the anterior cusp of the inferior molars 

 approaches what is seen in Amhlydonus. The small transverse posterior 

 superior molar of Triisodon further distinguishes it from Amblycfonus. A 



'Proceedings Academy Pliiladelpliia, 1875 (November 30). 



