838 THE MIOCENE FAUNA. ' 



This squirrel was about as large as the existing Sciurus niger. It was 

 common during the White River epoch, specimens having been obtained 

 in Dakota, Nebraska, Wyoming, and most abundantly in Colorado. 



CASTOR Linn. 



Syst. Nat. I, p. 78, 1766.— Cope, Bulletiu U. S. Geol. Surv. Terrs., VI, p. 368. Steneofiber, E. Geoffr., Revue 

 Encyclop^dique, 1833. — " Chalicomyn Meyer, Nenes .Tahrbuch, 1838, p. 404, et 11^46, p. 474." — 

 Palceocastar Leidy, Extinct Jlamnialia Dakota JuiU Nebraska, 186y, p. 338. 



The family of the Castoridce differs from the Sciurklce in the absence of 

 postorbital angles or processes, and the presence of a prolonged tube of the 

 meatus auditorius extemus. In both of these points it agrees with the Hap- 

 lodontiidce, a family which Mr. Alston has distinguished from the Castoridce 

 on various grounds. I do not think any of his characters are tenable, ex- 

 cepting that drawn from the form of the mandible, which is expressed thus 

 in Mr. Alston's diagnosis : "Angular portion of mandible much twisted " 

 I have described this character better as follows : Angle of mandible with 

 a transverse edge due to inflection on the one hand, and production into an 

 apex externally ; the inflection bounding a large internoposterior fossa. 



Mr. Alston enumerates four genera of Castoridce — Castor, JJiohroticus, 

 Steneofiber, and Castoroides. J. A. Allen has shown that the last-named 

 genus cannot be referred to this family. The characters of Diobroticus, as 

 given by Alston, are as follows : " Skull much as in Castor. Third upper 

 molar and lower premolar elongate, with four enamel folds, the rest with 

 only two ; all the folds soon isolated." This diagnosis appears to separate 

 the genus satisfactorily. The definition of Stemofiber is as follows : " Parie- 

 tals not parallelogrammic ; interparietal subhexagonal ; basioccipital not 

 concave ; grinding teeth as in Castor, the subsidiary folds sooner isolated." 

 The distinction from Castor here rests exclusively on the forms of the parie- 

 tal, interparietal, and basioccipital bones. This kind of definition is always 

 of questionable validity, as the terms " parallelogrammic," " hexagonal," 

 &c., are not intended to be exactly used and cannot be exactly applied. 

 The Castor (Steneofiber) peninsulatus illustrates this fact, for there is no strik- 

 ing difference in the forms of the two bones to which these terms are ap- 

 plied, as compared with the Castor fiber. The basioccipital bone differs from 



