186 Field Columbian Museum — Geology, Vol. i. 



ternal cleft is traceable in the elongate third fossa, which is barely 

 separated from the enamel border. A mere trace of a fourth 

 fossette lying lateral to, and between the second and third, doubt- 

 less represents the posterior one in the ancestral form. The true 

 molars have lost their important characteristics in the process of 

 reduction, but even in the earlier form the first molar shows the 

 antero-posterior compression so noticeable in Mesogaulus, while 

 in that form the second molar retains something of the diagonal 

 elongation seen in the other, and also a trace of its median inter- 

 nal flexure. 



The following table will be of service in showing the distribu- 

 tion, both in time and locality, of the extinct sciuromorph rodents. 

 It is based upon Zittel's classification, except in instances where 

 later discoveries have thrown additional light upon the subject: 



