P: 
1893.] Succession of the Teeth in Mammals. 507 
being drawn at the first molar, for upon the molars rested the 
necessity of complex development, and such development was 
best effected in permanent crowns. 
1. All the so-called “milk molars” plus the so-called “true 
molars” constitute the first series. Beneath one or more of the 
“true molars” in lower mammals are rudiments of a second 
series. The second series consists therefore of these sub-molar 
rudiments plus the successional or permanent premolars, 
incisors and canines. 
2. In the stem Marsupials the entire first series persisted 
and became mainly permanent (non-deciduous); the second 
series became rudimentary and non-successional with the 
exception of the fourth upper and lower premolars and 
possibly one or two other teeth which either replaced or were 
intercalated between members of the first 
series. One or more premolars were suppressed and one 
more molar retained than typical in the Placentals. Thus is 
explained the apparently atypical dental formula of Marsupials. 
3. In the stem heterodont Placentals (excepting the Cetacea 
and Edentata) the entire first series persisted and all the 
incisors, canines and premolars remained deciduous. The 
successional second series persisted as far back as the first 
molar. r 
4. In the stem Cetacea the entire first series persisted and 
the second series became rudimentary and non-successional. 
The tooth form changed from a heterodont to a homodont 
type. 
5. In the stem Edentates, which also transformed from the 
heterodont to the homodont type, the first series became rudi- 
mentary and the second series persisted in the succession even 
behind the region of the first molar. 
Finally, there is evidence that a primitive succes- 
Sion inthé region of the molarteeth, lost 
1The law of molar evolution is that complication is most rapid in teeth which are 
longest in use. Thus the first molar is the most progressive tooth of the true molar 
series and the last premolar is the most progressive of the premolar series. The 
apparent exception that the third milk premolar is always an advance type of the third 
permanent premolar is = by the fact that the milk premolars are formed to 
assume the molar functio 
