244 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



This division embraces the many types observed in the Ungu- 

 lates, some Rodents, and possibly Garni vora. Inasmuch as the 

 teeth of the maxillary and dentary (mandibular) bones do not al- 

 ways conform to the same type (e. g., Symborodo7i, Equus), it 

 will be necessary to consider them separately. Besides the differ- 

 ence in type, they differ in their rela- 

 tive development in width in the 

 more specialized forms ; thus, in 

 Homo, Mus, Mastodon, and such 

 genera, the molars of both jaws are 

 identical ; in Palceotlierium, Eohasi- 

 leus, Tcqnrus, etc., and most Car- 

 nivora, the superior are the wider, 

 the inferior narrower, appropriately 

 to the greater slenderness of the 

 mandibular bone. The latter, or 

 anisognathous type, may then be regarded as the more specialized. 

 The Bunodonts, except some Carnivora, are all of the former or 

 isognathous type ; among Lophodonts the few Rodents, the Dino- 

 theriidcB, and Elephas are isognathous, while all of the other 

 Prohoseidia, the Perissodactyla, and Rumina7itia are anisogna- 

 thous. Examples may be selected as follows : 



Isognatlious ; Bunodonts : Homo, Dicotyles, Sus, Mastodon, 

 Elotlieriiim ; Lophodonts : Dmotheriidce, Eleplias ; Rodentia, 

 JSciuridce. 



Anisognatlious ; Bunodonts : Macacus, Lemur oidea, Procyon ; 

 Lophodonts : A7iopIotherium, Hyoj^otamus, Oreodon, Ruminantia, 

 Perissodactyla, Batlimodon, TJintatlierium, 



Fig. 18. — BJiinocerus (temporary). 



II. THE subordi:n"ate types of bui^odojs^ts. 



1. The Maxillary Teetli.^ » 



It is apparent that the type of superior molar tooth which pre- 

 dominated during the Puerco epoch was triangular or tritubercu- 

 lar ; that is, with two external and one internal tubercles, f Thus, 

 of sixty-seven species of placental Mammalia of which the suj^erior 

 molars are known, all but four have three tubercles of the crown, 



* This division is inserted from the " Proceedings of the American Philosophical 

 Society," for December, 1883, for the sake of supplying an omission in the original 

 memoir. (Ed. 1886.) 



f See "American Naturalist," April, 1883, p. 407. 



