592 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



as high in young and medium-sized specimens, angular on the outer side, 

 but becoming more rounded in large examples; surface unknown. 



Height of largest specimen seen, 0.30 inch ; greatest breadth of same, 

 0.40 inch/ 



I only refer this species doubtfully to the genus Valvata, not having 

 seen any good specimens of it. It has its volutions more compressed on the 

 upper and lower slopes, and its periphery more angular than is common in 

 that genus; while its umbilicus is comparatively small for a Valvata. 



Locality and position. — Judith River Fresh- and Brackish-water Lignite 

 beds, at the mouth of Judith River, on the Upper Missouri, Montana ; prob- 

 ably belonging to the latest member of Cretaceous. 



