246 THE STRUCTUKAL EVIDENCE OF EVOLUTION. 



ing at opposite angles of a triangular space ; the outer and ante- 

 rior cusps are the most elevated, and the ridge which connects 

 them is now a cutting blade. The posterior portion of the tooth 

 does not share in this elevation, and its two tubercles are in some 

 genera obsolete, and in others replaced by an elevation of one mar- 

 gin, which leans obliquely toward the middle of the crown. In 

 Mesonyx this is represented by a median longitudinal crest. If 

 the two tubercles of the posterior part oi this tooth (which may be 

 termed a tubercular seciorial) are elevated and acute, we have the 

 molar of many recent and extinct Insectivora ; if the same por- 

 tion (now called a lieel) is much reduced, we have the type of 

 Oxycena and Stypolo2)lius. In the Canidce the three anterior tu- 

 bercles are much less elevated than in the genera above named ; the 

 external is much the larger, and the anterior removed farther for- 

 ward so as to give the blade a greater antero-posterior extent. 

 The heel is large and without prominent tubercles. In the Muste- 

 lidm the inner of the two median cusps is often reduced to a rudi- 

 ment, or is entirely wanting, and the heel is large. The lower 

 sectorial of the HycenidcB has no inner tubercle, and the heel is 

 much reduced. In some of the saber-toothed tigers the heel re- 

 mains as a mere rudiment, while in the true cats it has entirely 

 disappeared, and the carnassial tooth remains perfected by sub- 

 traction of parts, as a blade connecting two subequal cusps. The 

 HycBnodontidcB, 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 inferior carnassial dentition has thus 

 been accomplished by the subtraction of the inner and posterior 

 cusps, so that of the original four of the quinquetuberculate molar 

 but a single one, i. e., the anterior external, remains. 



III. THE SUBORDIjq-ATE TYPES OF LOPHODOXTS. 



1. The Maxillary Teeth, 



In the essay already quoted * the following remarks (page 7) 

 explain the relation between the Bunodont genera and several of 

 the Lophodont types of superior molar teeth : "In the superior 



* "Primitive Types of Mammalia Educabilia," May, 1873, and Hayden's "Re- 

 port on Geological Survey of Montana, Wyoming, etc.," ISYS, p. 646. 



