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University of California. 



[Vol. i. 



same age as the Berkeley Pliocene, there is found, perhaps, another 

 exception. The specimen figured by Leidy,* Fig. 2, a, shows only 

 three molars on each side, and in his description no reference is 

 made to a fourth. The figure shows what is perhaps an alveole 

 for a fourth tooth on the right side, but no mention is made of it in 

 the text. The great reduction in size of the posterior molars would 

 lead one to suppose that the third molar had disappeared in Eneas- 

 tor. It is, however, true that in Trogonthcriinn, Fig. 2, c, in which 

 molars I and 2 are also much smaller than the premolar, a third 

 molar, larger and more complex than the others, is present. 



Figure 2. — a, Upper molar series, right side, of Eucastor tortus; J, left, upper 



premolar of the same {a and a twice natural size), after Leidy. 

 />, upper molar series, left side, of Sigmogomphius Le Contei; 

 b' y section of right upper premolar, cut a short distance above 

 the triturating surface {b and b' twice natural size); c, left su- 

 perior molar series of Trogotithcriitm Cuvieri^ (natural size), 

 after Newton. 



In number and arrangement of the enamel folds SigmogompJiins 

 differs from Castor and Steneofiber, but approximates the charac- 

 ters in Eucastor and Trogontlierium. In the first two genera there 



^Extinct Mammals of Dakota and Nebraska, PI. XXVI, Fig. 21. 



tThis figure was copied from an illustration, in which the enamel folds 

 were indistinctly shown on the outer, right side. It was therefore impossible, 

 in some cases, to tell whether the loops were closed or open at the outer end. 



b 



c 



