; 552 The American Naturalist. [July, 
have epitomized the slow transformation of the single-fanged 
conical reptilian tooth, such as we see in the serpents, into the 
low-crowned human grinder. We now know all the transi- 
tion forms, so that we can homologize each of the cusps of the 
human molar with its varied ancestral forms in the line of 
descent. For example, the anterior lingual or inner cusp of 
the upper true molars traces its pedigree back to the reptilian 
cone. The anterior triangle of cusps, or trigon, seen in the 
mosozoic mammalia, and persisting in the first inferior true 
molar of the modern dog, is still seen in the main portion of 
the crown of the human upper molars (pr, pa, me). To this 
was added, ages ago, the posterior lingual cusp, or hypocone, 
which, as Cope has shown, is exhibited in various degrees of 
development in different races and is an important race 
rigon- 
Lower molar. Bi paeen r and lower molars opposed. Upper molar. 
Kev To Pian A UPPER AN MOLARS IN ALL MAMMALS.—Each tooth oe of a 
triangle, ¢rigon, with the protocone, fr, at the met, The apex is on the inner side of the upper 
molars and on the outer ike of the lower molars 
index. A glance through the diagrams shows that the 
development of the crown has been by the successive addition 
of new cusps. Without entering upon the details of evidence 
which would be out of place here, I may say briefly that the 
new main cusps have developed at the points of maximum 
1The upper molars in many Esquimaux are triangular (as in ten 11); in most 
negroes they are square (Fig. 12). In our race they are intermediat 
