No.2.] THE HARD PARTS OF THE MAMMALIA. 227 
the nutrition of the parts probably has had an important influ- 
ence on the process. Each basal cusplet stands nearer to the 
point of entrance to the crown of the nutritive artery which 
ascends — or descends through the root —than any other cusp- 
let, and would therefore grow more rapidly than any other 
secondary part of the crown under the stimulus of use. The 
basal cusplets have thus replaced those occupying more elevated 
positions on the principal cusp, and ultimately in some groups 
equal it in dimensions; as in the true inferior sectorial of Car- 
nivora, and in quadritubercular types. 
5. THE ORIGIN OF THE TRITUBERCULAR MOLAR. 
The anterior cusplet of the triconodont crown has been called 
in the upper jaw the paracone, and in the lower jaw the para- 
conid ; and the posterior cusplet is the metacone or metaconid, 
respectively. These cusplets serve to fill up the spaces between 
the teeth, and thus to produce a certain amount of interference 
Figure 51.— Tricondon ferox Marsh, inner side of ramus; three times natural 
size: g, mylohyoid groove. 
figure 52.— Menacodon rarus Marsh: a, outer, c, inner, side of ramus, three 
times nat. size; all from Jurassic beds of Wyoming; from Marsh. 
