Sigmogomphius Le Contei. 



369 



are always two or more enamel folds in the outer wall of the supe- 

 rior molars; in Eucastor, Trogontherium, and Sigmogomphius, the 

 normal number for molars 1 and 2 is one. Both outer and inner 

 folds are more oblique in these three forms than in Castor and 

 Steneofiber. 



Compared more particularly, Fig. 2, with Trogontherium and 

 Eucastor, which are evidently its nearest relatives, Sigmogom- 

 phius differs from the first in having three molar teeth instead of 

 four, in the more nearly quadrate form of molars 1 and 2, and in 

 the open character of the folds, those of Trogontherium being in 

 part closed up, forming islands. From Eucastor it differs again in 

 possessing open folds, instead of the islands so characteristic of 

 that genus. From both it differs in the relatively greater size of the 

 molars. In Trogontherium and Eucastor the triturating surface of 

 the premolar is nearly as long as that of molars 1 and 2 together, 

 while in Sigmogomphius the length of the premolar as compared 

 with that of molars 1 and 2 combined is about as one to one and 

 a half. 



From the above comparison it appears that the new form is 

 more closely related to Eucastor and Trogontherium than to any- 

 other forms; also that the differences between it and these genera 

 are too great to permit its reference to either one. 



GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION OF THE CASTORII >.E. 



In the following table, giving the geological distribution of 

 most of the known Castoroid species, the American Castoridae 

 seem to reach their maximum development at or before the begin- 

 ning of Pliocene time. If the culmination of the group in Eurasia 

 can be definitely located at all, it would seem to occur in the Plio- 

 cene, or somewhat later than in America. This seems the more 

 probable, as the genus Steneofiber lived longer in Europe than in 

 America, while of the three related genera, Eucastor, Sigmogom- 

 phius, and Trogontherium, the American forms became extinct 

 long before the European. 



The apparent earlier culmination of the American Castoridae, 

 together with the earlier extinction of certain forms in this country, 

 seem to point toward the American rather than the European 

 origin of the family. 



