Douglass : Vertebrates from Montana Tertiary 



181 



Fig. 20. N'ertebra of 

 fish (No. 857). Loup 

 Fork, Lower Madison 

 Valley. End view, 

 natural size. 



large fish. It was found in the beds of sand and gravel about nine or 

 ten miles south of Logan. 



Mm. 



Length of centrum 21 



Width " " 19 



MAMMALIA. 

 RODENTIA. 



Sciurus sp. 

 No. 746. Fig. 21. 

 Part of a left mandibular ramus from the Loup 

 Fork of the Lower Madison Valley. 



This is about the size of the corresponding jaw 

 of the black squirrel. The most noticeable pe- 

 cularities are the prominence of the ridge for the attachment of the 

 inner part of the masseteris lateralis muscle, the anterior position of 

 the anterior angle of the masseteric fossa, the length of the mandi- 

 ble anterior to the molars, and the anterior position of the mental 

 foramen. The lower border of the anterior portion of the jaw as far 

 back as the beginning of the angle is the arc of a circle. 



The length of the molar-premolar series is less than in S. vortniani, 

 and the mandible is not so deep. The premolar is the smallest tooth 



of the series, and the molars are nearly- 

 equal in size. The teeth are oblique as in 



Arctoniys. 



Measurements. 



^'<i^^K 



''^; 



Length of incisor to back of M^ 24 



Length of molar premolars 9 



Fn;. 21. Sciurus sp. 

 (No. 746). Loup Fork, 

 Madison Valley. Natural 

 size. 



Sciurus arctomyoides sp. nov. 

 No. 741. Fig. 22. 

 A small portion of a skull with the in- 

 cisors and three anterior cheek teeth and 

 the greater ])art of a mandible. It was 

 with the skeleton of Pahcarctomys montainis in the Loup Fork beds 

 of the Lower Madison valley. 



The upper incisors have a slight median longitudinal depression on 

 the anterior surface, which is also covered with minute interrupted 

 striations. The anterior premolar is minute. The last premolar is 



