22 PROCEEDINGS OF THE ACADEMY OF 



and the heel is large. The lower sectorial of the Hysenidae has 

 no inner tubercle, and the heel is much reduced. In some of the 

 sabre-toothed tigers, the heel remains as a mere rudiment, while 

 in the true cats it has entirely disappeared, and the carnassial 

 tooth remains perfected by subtraction of parts as a blade con- 

 necting two subequal cusps. The Hyaenodontidse, as is known, 

 possess three carnassial teeth without inner tubercles. The history 

 of this form is as yet uncertain, as it was evidently not derived 

 from contemporary forms of the Eocene with tubercular sectorials. 



The development of the carnassial dentition has thus been ac- 

 complished, first by an addition of anterior cusp, and subsequently 

 by the subtraction of the inner and posterior cusps, so that of the 

 original four of the quadrituberculate molar but a single one, i.e., 

 the anterior external, remains. The same process may be observed 

 in the successional modifications of the entire dentition of the 

 jaws. The Eocene forms of Carnivora frequently display more 

 numerous sectorial teeth (such as they are) than any of the ex- 

 isting families. The important change which is clearly indicated 

 is the progressive extinction of the genera with numerous sectorial 

 teeth, accompanying the increasing specialization of the sectorial 

 tooth in the genera which remain. In other words, the numerous 

 types of digitigrade carnivora which have survived, are those 

 developing but one sectorial tooth (whose earliest representative 

 is Didymictis). The increased perfection of the sectorial has been 

 associated with a reduction in the number of other molars, first 

 posterior, then anterior to it, which reduction has been accompanied 

 by an increased relative size of the sectorial. By this process 

 concentration of the carnassial function has been gained, and in- 

 creased robustness of the jaws, by progressive shortening. The 

 slender form of the rami of the Eocene genera and Hysenodon, are 

 much less efficient in functional use than the stout jaws of existing 

 Mustelidae, Hysenldse, and Felidse. 



A second point in the history of the Eocene Carnivora remains 

 to be considered. In all of the genera which I have had the 

 opportunity of examining, excepting Mesonyx, namely, Ambloc- 

 tonus, OxysencL) Prototomus, and Didymictis, the tibio-astragalar 

 articulation is of a primitive character. The astragalus is flat, 

 and the applied surfaces are nearly plane, and without the pulley- 

 shaped character seen in existing Carnivora, as dogs, cats, and, in 

 a less degree, in the bears, and in other Mammalia with specialized 

 extremities, as Perlssodactylci , Artiodactyla, etc. The simplicity 

 of structure resembles, on the other hand, that found in the op- 

 possum, and various Insectivora, Rodentla, and Quadrumana, and 

 in the Proboscldia, most of which have the generalized type of 

 feet. The structure indicates that the carnivorous genera named 

 were plantigrade, a conclusion which is in conformity with the 

 belief, already expressed, that the Mammalia of the Eocene exhibit 

 much less marked ordinal distinction than do those of the Miocene 



