1892.] NATURAL SCIENCES OF PHILADELPHIA. 419 



except the first, increasing in size as we pass from before backward 

 and thus corresponding with the development of the deuterocone in 

 the upper premolars. A very similar statement will apply to 

 I*alceonicti<. 



The fourth premolar in both jaws exhibits great variety among 

 creodonts from a completely molariform to an extremely simple 

 condition. Taking first the upper tooth, we find in such Puerco 

 genera as Oxyclcenns and Deltatherlinn, the simplest which occurs 

 among the creodonts, viz., a compressed, acute protocone, with 

 trenchant anterior and posterior edges and a conical deuterocone. 

 From this stage several distinct lines may be traced in as many 

 diverging directions. Proviverra agrees with Deltatherium in the 

 construction of this tooth, but in Sinopa (Stypolophus) it is compli- 

 cated by the addition of a small tritocone and in some species, of 

 an anterior basal cusp, or protostyle, as well. Sinopa has thus 

 reached a stage of development of P4 in which all the elements of 

 the carnivorous sectorial are present, and yet the small size of the 

 tooth and especially the incomplete development of the tritocone 

 render the term sectorial inappropriate to such a tooth. In some of 

 the Hycenodontid(e, however, as Oxycena for example, P4 fully 

 deserves to be called a sectorial, as the compressed protocone and 

 enlarged trenchant tritocone form a very efficient cutting blade. 

 But even here the main emphasis of development is on the first 

 molar, which is larger and of more functional importance than the 

 premolar. In this genus. Pa, also has all the elements of a sectorial. 

 In Hycenodon itself, P4, is reduced in size and importance, m^ being 

 the largest and most efficient tooth in the series ; the protocone is 

 large, but the tritocone is insignificant, and the deuterocone so 

 diminished that in some species it can hardly be said to exist as a 

 separate element. Palceoiiidi-", though obviously belonging to a 

 phylum very distinct from that of the hyseuodonts, closely resembles 

 OxycBua in the condition of the superior premolars. 



The Miacid(B alone among creodonts have attained the carni- 

 vorous type of dentition. The superior sectorial is not, it is true 

 more efiiciently developed than in Oxycena, but, on the other hand, 

 the upper molars are tubercular, not sectorial at all. In Didymic- 

 tis P4 is quite like that of the viverrines, owing to the presence of a 

 small, but very distinct anterior basal cusp. 



A very different type of premolar tooth is that assumed by P* in 

 the Mesonyclddce. Though the same elements are present as in 



