170 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Vol. YI. 



The interval between them and the Nimravidce is, however, great, for in 

 the Oxycenidce, when there is a superior sectorial tooth, the first true molar 

 in the upper jaw is utilized instead of the last premolar, and the second 

 true molar below is a sectorial as well as the first. Several intervening 

 forms must yet be found to complete the connection, if it have ever ex- 

 isted. It is, however, very likely that the true FeUdce were derived 

 from the genus Procelurus, through Pseudcelurus, if indeed these two 

 genera be not the primitive members of that family, for, as above re- 

 marked, the evidence of their i)ossession of the characters of the Nhn- 

 ravidce has not yet been obtained. There can be no reasonable doubt 

 that the genera Drepanodon and Smilodon in the FeUdce are the descend- 

 ants of SoplopJioneus and allied genera. In fact, the ISfimravidce and 

 FeUdce are ^' homologons groups ", having corresponding terms in the 

 manner I foreshadowed as a general i)rinciple in 1868 (Origin of Genera). 



In looking for causes in explanation of the modifications of structure 

 cited, one can easily discover that there is a close relation between the 

 arrangement of the teeth and the mechanical laws involved in the per- 

 formance of their function, that of seizing an active prey and of cut- 

 ting up their carcasses into pieces suitable for swallowing. It is obvious 

 that in the latter case the flesh-teeth bear the resistance and the masseter 

 muscle is the power, and that the nearer these parts are together the 

 better is the function performed. As a matter of fact, the sectorial teeth 

 in modern Garnivora are placed exactly at the angle of the mouth, which 

 is nearly the front border of the masseter muscle. 



Both the muscle and the teeth have, however, moved forwards in con- 

 nection with the shortening of the jaw behind. This has been due to 

 the necessity of bringing the power (masseter) nearer to another point 

 of resistance, viz, the canine teeth. In the early carnivores (as 

 Hycenodontidce) the long jaws supported more numerous teeth {\^) 

 than in any modern families, and the fissure of the mouth was probably 

 very wide. The canine teeth were evidently very ineffective weapons. 

 The animals probably only snapped with their jaws, and did not attempt 

 to lacerate or hold on, as do the cats. The dogs of to-day are long-jawed, 

 and they snap in a manner quite distinct from anything seen among 

 the cats. The only dogs that hold on are the short-jawed bulldogs. 



So in the use of the canines, we have the ground of the shortening 

 of the jaw behind and before, and the consequent change of structure, 

 which resulted in the modern perfected FeUdce. 



