370 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Tol. VI. 



Some affinity probabty exists between this family and the Mylagaulidcs, 

 which followed in the Loup Fork epoch. 



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

 the G. nebrascensis* from the White Eiver, nor the occipital bone of the 

 C. pansus, t from the Loup Fork formation. 



I. Basioccipital bone deeply concave below : 



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. j)eninsidatus. 



Palate narrower ; size small C, gradatus. 



Ci\STOE PENINSULATUS Cope. 



Steneofilier f nebrascensis Leidy, Cope, Bulletin U. S. Geological Survey Terrs. V, 

 1879, p. 55. 



This species is about the size of a large prairie marmot — Cynomys ludo- 

 vicianus. It was abundant in Oregon during the period of the Truckee 

 Miocene. Leidy originally described the closely allied C. nebrascensis 

 from the White Eiver beds of Nebraska, but I have never obtained it 

 from that formation. Another and similar species, G. pansus Cope, is 

 common in the Loup Fork beds of Kebraskaand jS'ew Mexico. 



Several well-preserved skulls from Oregon display characters not vis- 

 ible in specimens heretofore collected, and which enable me to make 

 fuller comparisons with the European G. viciacensis^ so fully described 

 by M. Filhol.t 



The postorbital constriction is much greater in this species than in 

 th.e C viciacensis^ and grea'ter than in the G. nebrascensis from the White 

 River beds. The straight anterior temporal ridges are in line with the 

 superciliary borders, and unite into a sagittal crest at the constriction. 

 In the 8. viciacensis they continue separate beyond this point one-fourth 

 the distance to the sui)raoccipital crest. The zygomata are wide, and 

 the malar ridge is very prominent anteriorly, overhanging the face, and 

 curving rather abruptly to the base of the muzzle. The latter is rather 

 wide, with parallel sides, and is flat above. The brain-case expands 

 lather abruptly from the interorbital constriction, and is rather flat 

 above. The infraorbital foramen is a narrow vertical oval and is sit- 

 tiated low down in the vertical line with the anterior extremity of the 

 malar angular edge. It is a little nearer the line of the first molar than 

 the posterior border of the superior incisor. The incisive foramina 

 are relatively larger than in the bieaver, and are chiefly in the premax- 

 illary l)one. T he palate between the anterior molars is as wide as the 

 transverse diameter of the first molar. There is no distinct fossa of the 



* Stcncofibm- nebrascensis Leidy, Proceed. Academy Pbila., 18'J6, 89 ; 1857, 89 ; Chali- 

 comi)S nebrascensis Leidy, 1. c, 1857, 176; Pahwcastor nc6rascens(s Leidy, Ext. Mamma- 

 lia Dakota and Nebraska, 1869, p. 338, sxvi, Figs. 7-11. 



] Steneofiber imnsus Cope, Report Capt. G. M. Wheeler, iv, pt. ii, 1877, p. 297. 



t Etude des Mamniiferes Fossiles de Saint-Grand-le-Puy, Allier. Bibl. de I'ficole 

 des Hautes Etudes, XIX, Art. I, p. 44, 1879. 



