Marsh Collection^ Peabody Museum. 367 



erable exactness. In connection with teeth having this devel- 

 opmental history, one important point to remember is, that the 

 antero-internal cusp, or the one which originates upon the 

 inner ridge of the posterior triangular area, is always slightly 

 posterior to the antero-external or main cusp. And it is also 

 of the utmost importance to recall that the apex of the original 

 single-pointed premolar corresponds to, and is homologous 

 with, the antero-external cusp. This has been determined as 

 true of the Ungulata, Carnivora, Insectivora, Primates, and 

 probably of other orders. 



In the case of the Rodentia, however, it is different. If a 

 perfectly unworn, lower fourth premolar of a member of the 

 Sciuromorph division is examined, figure 119, it will be seen 

 that the new cusp, instead of originating upon the internal, is 

 an outgrowth of the external^ descending ridge bordering the 

 posterior triangular area. It thus happens that the cusp which 

 corresponds to, and is homologous with, the apex of the origi- 

 nal single-pointed premolar is the antero-internal and not the 

 antero-external citsp^ as in the orders jnst referred to. 



119 



^ 



Figure 119. — Fourth lower premolar of a species of Paramys ; outside (a) 

 and crown (&) views ; three times natural size. 



Further proof of this is found in the fact that the antero- 

 internal cusp has a position in advance of that of the antero- 

 external, which should be the case if the new element had 

 arisen upon the external instead of upon the internal ridge. 

 The ancestral type of the Sciumorphs is represented by the 

 genus ParajJiys of the Eocene, figure 119, and in this group 

 the manner of origin of the premolar cusps is clearly shown. 

 Distinct traces of this succession are still visible in the squir- 

 rels and spermophiles of the present day. The genus My sops 

 of the Bridger beds {Eomys of the European Eocene), while 

 closely allied to Par amy s^ without much doubt represents the 

 beginning of the JVIyomorph division of the Rodentia, and it 

 is interesting to note that this same plan of origin of the 

 cusps of the lower premolars is true of this group as well. 

 No sufficiently primitive stages of the teeth of either the His- 

 tricomorphs or the Lagomorphs have to my knowledge as yet 

 been found, which would enable one to say with absolute 

 certainty whether or not the complication of their teeth has 



