8 
proceed so far as to produce on wearing a confluence of the erescentoid 
surfaces. This is the case in Orothertwm sylvaticum in the mandibular 
series. In both Paleosyops and Hyrachyus those tubercles of the upper 
molars are confluent into two Vs (more or less open, when unworn). In 
the former the inner tubercles retain their primitive conic tubercular 
form, but in Paleothertum, Rhinocerus, Lophiodon, Hyrachyus and Ta- 
pirus they elongate transversely so as to meet the corresponding outer 
tubercles (now crests) forming the familiar cross-crests of those genera. 
If the tubercles are alternate, they produce the oblique crest of Palao- 
therium, if opposite, the cross-crest of Tapirus. 
If on the other hand the inner tubercles flatten like the outer, on wear- 
ing, we have the quadricrescentoid type of Anoplotheritum and the Rumi- 
nants. 
But it is important to observe, that the lower types of Quadrumana 
and Carnivora present the quadrituberculate crown with tendency to 
flattening of the outer tubercles, as seen in these lowest Ungulatu. In 
the Carnivora the sectorial tooth is produced by the greater flattening 
and partial confiuence of the outer tubercles, and the, entire loss of the 
inner, the ‘“‘heel”’ being in the dogs and cats, ¢. g. their only representa- 
tive. In the Quadrumanous families, including man, the primitive quad- 
rituberculate type of molars is preserved, the flattening of the outer 
tubercles being finally lost. 
It is to be observed that the lines of Ungulata, Quadrumana, and Car- 
nivora, originate in plantigrade types, a state of things quite predomi- 
nant among the lower series, or Lissencephala. It is universal in Hden- 
tata and very usual in Rodentia and Insectivora. The lower forms of 
Moarsupialia and all of the Monotremes present it. In the Marsupials, 
Rodents, Ungulates, and Carnivores we have series whose highest ex~- 
pression is in the most highly digitigrade genera. 
Tbe accompanying diagram is designed to express to the eye more 
clearly the propositions made above. By comparing it with a similar 
table published by Prof. Gill (Proceedings of the American Association 
for the Advancement of Science for 1871, p. 295), a close resemblance 
between the two may be observed, as well as certain differences. 
I wish to be understood that the genera named in it as ancestors, are 
to be regarded in the light of types of groups. There is no other mode 
of explaining the facts, than that in accordance with the law of ‘““homolo- 
gous groups,’ ¢.¢. that several genera of one group have undergone simi- 
lar modffication into corresponding ones of a second group.* 
* See Origin of Genera, page 79, Prop. V. 
Published, May 6th, 1878. 
