MCERITITERIUM. 
107 
posterior diameter of the base of the tusk is about 2*8 cm., the lateral diameter the 
same. The third incisor (^. 3) was a small tooth, nearly circular in section, and 
situated immediately beliind the tusk, just internal to its outer angle. This tooth is 
represented by its alveolus only in all specimens examined ; the antero-posterior 
Cast of the cranial cavity of Mastodon americanus : A, from above ; B, from right side. 
(•6., cerebellum ; f.l., frontal lobe ; p., pseudosylvian depression ; o.l., olfactory lobe ; t.l., temporal lobe. 
