SCIUROMORPHA. 839 



that of the beaver, but not so as to conform to J. W. Alston's diagnosis ot 

 the genus 8teneofiber. Its inferior surface is concave, but doubly so, as a 

 keel occupies the median line. In the -S^. viciacensis according to Filhol, this 

 region is shallowly concave, without median keel. Although important as 

 specific characters, these variations do not appear to me to require the rec- 

 ognition of as many genera. The possession of the epitrochlear foramen in 

 the S. viciacensis is at first sight an important character. Mr. Filhol, how- 

 ever, informs us that of thirty-four humeri which he has studied, sixteen 

 possess the foramen, and in eighteen it is wanting. 



The Castor tortus was described by Leidy from the Loup Fork forma- 

 tion. He coined the subgeneric name Eucastor for it without corresponding 

 definition. In his monograph of the Castoridce, J. A. Allen referred this 

 species^ to a genus distinct from Castor, and defined it, using for it Leidy 's 

 name Eucastor. This genus appears to me to be valid. The three genera 

 of Castoridce will then be defined as follows : 



Molars and premolars with one inner and two or three outer folds Castor. 



" Inferior premolar and third superior molar elongate, with four enamel folds ; the rest 



with only two " Diohroticus. 



Superior premolar enlarged, with one inner fold ; inferior molars small, with two 



lakes Buca.stor. 



Some affinity probably exists between this family and the MylagauUdce, 

 which followed in the Loup Fork epoch. 



The species of Castor may be distinguished as follows. I do not know 

 the C. nebrascensis^ from the White River, nor the occipital bone of the C. 

 pansus^ from the Loup Fork formation. 



I. Basioccipital bone deeply concave below, simple ; 



Angle of lower jaw not deflected C. fiber. 



II. Basioccipital shallowly concave below: 



Angle of lower jaw deflected C viciacensis. 



III. Basioccipital concave below with a median keel; angle of lower jaw not deflected. 



Palate wider ; size medium C. peninsulatutr. 



Palate narrower ; size small C. gradatus. 



' Monographs of North American Rodentia, Coues and Allen, U. S. Greol. Surv. Terrs., 1877, xi, p. 450. 



'^ Steneojfber nebrascensis Leidy, Proceed. Academy Phi la., 1856, 89; 1857,89; Chalicomya nebraseeiisis 

 Leidy, 1. c, 1857, 17(); J'alaocaetor nehrascensis Leidy, Ext. Mammalia Dakota and Nebraska, 16U9, p. 

 338, xxvi. Figs. 7-11. 



Steneofiber pansus Cope, Report Capt. G. M. WTieeler, iv, pt. ii, 1877, p. 297. 



