EXPLANATION OF PLATE XXI. 



Figs. 1-4. Mastodon obscurus: 



Last lower molar of the left side, natural size. Specimen discovered by Dr. Lorenzo G- 

 Yates, in Contra Costa County, California - , and now in the museum of Amherst College. 



Fig. 1. View of the triturating surface. 



Fig. 2. Outer view of the same specimen. 



Fig. 3. Fragment of a tusk, two-thirds the natural size, exhibiting the broad band of enamel 

 indicated by the darker shade. Specimen found by Dr. Yates in Stanislaus County, Cal- 

 ifornia, and belonging to Amherst College museum. 



Fig. 4. Outline of the transverse section from the smaller end of the same specimen, of the 

 natural size. 



