MOLAR TEETH OF THE MAMMALIA EDUCABILIA. 261 



similar to those already indicated among BatracMa, Anura, CepJi- 

 alopoda, etc.* That this relation indicates descent of the corre- 

 sponding terms of the one series from those of the other has also 

 been rendered highly probable. This conclusion has also been 

 previously stated as a theorem, f as follows : ^' V* The heterologous 

 terms or genera in the later series are modified descendants of those 

 of the earlier series " ; in other words, that certain groups higher 

 than genera are produced from others of a similar high value by 

 *^ descent with modification." 



As already pointed out, the Bunodont primary genera belong to 

 the older geologic epoch of the Eocene, while most of the deriva- 

 tive ones belong to later periods. Some were contemporary with 

 the primary forms, but doubtless have descended from pre-existent 

 members of the same type as yet unknown to us. The genus 

 Achmnoclon, Cope, is especially generalized in three respects : (1) 

 the simplicity of the construction of its molars ; (2) the same 

 simplicity of the premolars, which are without inner or posterior 

 lobes ; (8) the absence of all diastemata and consequent continu- 

 ity of the dental series. Hence it maybe regarded as more primi- 

 tive than PalcBOchcerus, Cheer omor us, Dicotyles, or Elotlierium, 

 in all which there are marked diastemata. The two series may then 

 be arranged as follows : with the understanding that in some cases 

 names of genera used represent rather family groups, in which 

 the special generic lines have not yet been made out.| 



The following table has been already published in its essential 

 features in the ^* Report on Geological Survey of the Territories," 

 1873, p. 648.* 



It remains now to ascertain whether the genealogical or taxo- 

 nomic relations expressed by the teeth coincide with those derived 

 from the other diagnostic regions of the body. First of these must 

 be selected, as of chief importance, the limbs and feet. 



* See " Origin of Genera," p. 53. t Lo<^- ^«^- P- '^9. 



X Since the following table was published it has become probable that Achaeno- 

 don is an unguiculate and a flesh-eater. It has also been discovered that the genus 

 Phenacodus possesses the characters which give it the place as the ancestor of all 

 the lines in the table. (Ed. 1886.) 



# It was previously published in a separate form in " On the Primitive Types of 

 Mammalia Educabilia," May 6, 1SY3. An error occurs in this edition in the reversal 

 by a lapsus calami of the positions of the types Omnivora and Anoplothcrium. It 

 is also important to note, that in the " Report G. S.," p. 645, where it is stated that 

 " during the Eocene they (the orders) were in process of difPerentiation," etc., Mam- 

 malia Educabilia and not Zissencephala, are referred to. 



