GEOLOGICAL SURVEY OF THE TEERITOEIES. 647 



the middle and posterior part of the mandibular series. In TJintatheriwii 

 the diagonal from the posterior crest never appears^ leaving a trans- 

 verse crest and a V on the true molars. 



In the superior molar series, the exterior flattening of the e:5terior 

 tubercles may proceed so far as to give crescentoid sections on wearing, 

 and their longitudinal extent may be such as to cause them to unite at 

 their bases. A similar succession of form may be seen in the inferior 

 molars, e. (/.,' in OrotJierium sylvaticum. In hoth Palcvo.syops mid Syra- 

 cJiyus, these tubercles (of the upper molars) are confluent into two Vs 

 (more or less open) when unworn. In the former, and in Limnohyus, the 

 inner tubercles retain their primitive conic tubercular form ; but in 

 FalccotJierium, Eliinoceriis Lopliiodon^ Myrachyiis, and Tapirvs,. they elon- 

 gate transversely so as to meet the corresponding outer tubercles, (now 

 crests,) forming the familiar cross-crests of those genera. If the tuber- 

 cles are alternate, they produce the oblique crest of Falceotherium; if 

 opposite, the cross-crest of Tajjirus. An interesting anuectant form is 

 seen in Oroliipims procyoninus^ where the two intermediate tubercles, 

 which separate the inner cones from the outer V's in Limnohyus, are so 

 developed as to constitute parts of an incomplete pair of transverse 

 ridges, which disappear in front of the bases of the oifter V's. These 

 represent the oblique crests of FalceotJierium and AncMtJiermm, and thus 

 the genus OroMppiis furnishes a station on the line from Palceosyops to 

 the horses. 



If, on the other hand, the inner tubercles flatten like the outer on 

 wearing, we have the quadricrescentoid type of Anoplotlierimn and the 

 Euminants. 



But ifc is important to observe that the lower types of Quadrumana 

 and Carnivora j)resent the quadrituberculate crown with tendency to 

 flattening of the outer tubercles, as seen in these lowest Ungulata. In 

 the Carnivora the sectorial tooth is produced by the greater flattening 

 and partial confluence of the outer tubercles, and the entire loss of the 

 inner, the " heel" being in the dogs and cats, e. g., their only represent- 

 ative. In the Quadrumanous families, including man, the primitive 

 quadrituberculate 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 Gar- 

 nivora originate in plantigrade types, a state of things quite predomi- 

 nant among the lower series, or Lissencephala. It is universal in Eden- 

 tata and very usual in Bodentio^ and Insectivora. The lower forms of 

 Marsupialia and ail of the Monotremes present it. In the Marsupials, 

 Eodents, Ungulates, and Carnivores, we have series whose highest ex- 

 pression is in the most highly digitigrade genera. 



The accompanying diagram is designed to express to the eye more 

 clearly the propositions made above. By comparing it with a similar 

 table published by Professor Gill, (Proceedings of the American Asso- 

 ciation for the Advancement of Science, for 1871, j). 295,) a close resem- 

 blance between the two may be observed, as well as certain diiferences. 



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 "homo- 

 logous groups," i. e., that several genera of one group have undergone 

 similar modification into corresponding ones of a second group.* 



* See Origin of Genera, page 79, Prop. V. 



