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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 MWesogaulus, 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 sczwromorph rodents. 
It is based upon Zittel’s classification, except in instances where 
later discoveries have thrown additional light upon the subject: 
