1897.] Trituberculy : 1013 
This method may be summarized as follows: Look for 
traces of primitive ancestral structure in the form and position 
of the cusps. Second, de- 
termine the divergent 
form, position, proportions 
and relations of the cusps. 
Third, determine the 
secondary cusps, crests 
and foldings, their form 
and relations. Finally, let 
us turn to a wholly differ- 
ent molar type and ex- 
amine the complex and 
aberrant molars of Coryphodon. Can we establish any homo- 
logies between its elements and those of any of the ungulates 
we have been considering? Fortunately we are partly guided 
by.the molar of the Puerco PERO E iaaii 
genus Pantolambda Cope, ) 
which is eyen older than 
the Coryphodons. This is 
our key to the ancestral 
or primitive form, and b 
Fig. 14.—Molars of Pantolambda, the 
ancestor of Coryphodon. 
parastyle-, 
rightly interpreted the 
homologies of the Cory- 
phodon molar elements. protocone~ 
We first note that nature 
has here evolved a lopho- —p”conid 
dont crown from the tritu- , 
bercular or trigonal basis, 
for there is no distinct 
talon or hypocone except 7 
in the unique form Mante- 
on.  Pantolambda has 
no parastyle, but a promi-  metsssnia n entoconid 
nent mesostyle and a pair Fig. 15—Molars of Coryphodon, showing 
of selenoid external cusps, Shifting of the Crests. | 
also a selenoid protocone with a spur leading toward a proto- 
A 
3 
