252 MAMMALIA. 



root. It is also noteworthy that the large fourth premolar 

 is often very low, its summit being elevated but little above 

 the true molars behind it. 



The family does not appear to have become extinct either 

 in Europe or North America until early Tertiary times, for at 

 least one genus (Neoplagiaulax) occurs in the Lower Eocene of 

 Rheims, France, and another (Ptilodus, fig. 148) in the Puerco 

 Eocene of New Mexico. The mandible in these genera is of 

 much interest because it exhibits the most extreme specialization 

 of the teeth in the family. In the French genus only the 

 fourth premolar remains, much enlarged, while in the American 

 genus (fig. 148) the same is observable, with only an insignificant 

 remnant of the third premolar. The two typical multituber- 

 cular molars are present in each. 



Another family (Polymastodontidae) is represented by 

 many detached teeth in the Laramie Formation and by more 

 satisfactory remains in the Puerco Eocene of the United States. 

 These are somewhat larger animals, with teeth arranged on the 

 rodent pattern, and the premolars are tubercular like the 

 molars. 



Polymastodon (fig. 149). The typical species of this genus, P. taoensis, 

 is known by a fragmentary skeleton equal in size to that of a large 

 kangaroo. There is a single pair of large incisors in both jaws, with 

 a diastema behind, and then follow three flattened grinding teeth, the 

 foremost relatively small and counted as a premolar. A minute pair of 

 lateral incisors is also present in the upper jaw. The crowns of the lower 

 molars bear two longitudinal series of stout tubercles, while those of 

 the two upper molars opposed to them are furnished with three similar 

 longitudinal series. The angle of the mandible is inflected, and the dental 

 foramen is at the anterior apex of a large fossa, as in most marsupials. 

 The form of the astragalus is believed to show that the inner digits of the 

 pes were much reduced in size, and that the outer digits were large, con- 

 stituting the principal agent in progression. The tail must have been 

 long. Puerco Eocene, New Mexico. 



It may be added that the bone-beds of the Laramie For- 

 mation, which have yielded so many teeth of the Multituber- 

 culata, have also furnished limb-bones, of which some present 

 interesting features. The pelvic bones are not fused into an os 

 innominatum, but remain suturally united. One scapula has an 



