1008 The American Naturalist. | December, 
cess can now be observed in a comparative series of recent 
Lemurs and Monkeys. To summarize the five steps toward 
the establishment of the ungulate primitive type: the addition 
of the lower talonid, the lowering of the cusps of the upper 
trigon, the addition of the upper talon and simultaneous 
lowering of the lower trigonid, the loss of the paraconid and 
hypoconulid. By these changes the cutting was transformed 
into the crushing type. The development of the talon 
necessitated the loss of the “ paraconid,” for they both occupy 
the same space when the 
jaws are closed; the stages 
of this gain to the upper 
molar and loss to the lower 
are well shown in the species 
of Euprotogonia. 
All these changes belonged 
to the constructive period 
and took place presumably 
before the great divergence 
of the ungulate orders began; 
or it may have been partly 
due to paralellism or homo- 
plasy, because we find that 
the molars of Trigonolestes, 
Fig. 11.—PREMOLAR TERMINOLOCY, the earliest known artiod- 
PROPOSED BY Scorr. Primitive Unga- actyl, are tritubercular. Some 
late Types. Fourth upper premolar and groups, such as those to 
first molar of A. Euprotogonia, and B. which Coryphodon, Uintather- 
Hyracotherium. j 
ium and Periptychus belong, 
built up their whole molar structure upon the tritubercular 
or trigonal basis. : 
From this point onward dated the period of “ moderniza- 
tion.” An important legacy of the old triangular form was the 
oblique arrangement of the outer and inner cusps parallel with the 
sides of the primitive triangles. Thus all the primitive crests 
developed upon these cusps were oblique and not directly 
transverse. The main features of modernization upon which 
we must now closely direct attention are: 
