54 CANADIAN PERIOD. 



beyond the liinge-line; cardinal angles rounded; liinge-line short, nearly- 

 straight ; lateral and front margins regularly rounded. 



Venti-al valve obliquely depressed-subconical; apex acute, prominent, 

 and perforated by a minute foramen ; margin in front of the hinge-line 

 regularly rounded ; area small, triangular, nearly flat, the angles which it 

 forms with the sides of the shell rounded. Sm-face of both valves smooth, 

 or marked by very fine concentric lines of growth. 



Width, two millimeters ; length a little less ; height a little less than 

 the length. 



This species, although so minute, seems to be a well-marked one, and 

 possesses all the visvial external characteristics of Acrotreta, except that the 

 ventral valve is not so capacious as it generally is in that genus. It differs 

 from A. gemma Billings, from strata of the age of the Quebec group in New- 

 foundland, in the less proportionate lieight of the ventral valve, and in the 

 absence of a mesial sinus in the dorsal valve. 



Position and locality. — Strata of the age of the Quebec grovip of Canada; 

 Schellbourne, Schell Creek range, Nevada. 



Order ARTHROPOMATA. 

 Family STEOPHOMENIDiE. 



Genus STROPHOMENA Rafinesqiie, 1827. 



Strophomena fontinalis White. 



Plate III, fig. 4 a, b, and c. 



Strophomena fontinalis Wbite, 1874, Expl. & Surv. west 100th Merid., Prelim. Rep. 

 Invert. Foss., 10. 



Shell moderately concavo-convex or nearly flat ; outline semi-elliptical ; 

 width from one-quarter to one-thu-d greater than the length ; width at the 

 hinge-line varying from a little more to a little less than it is just in front 

 of the hinge. Ventral valve slightly convex or somewhat flattened ; con- 

 vexity greatest behind the middle. Dorsal valve slightly concave, and in 

 other respects corresponding with the ventral. Hinge and interior of both 

 valves unknown. 



Surface of both valves marked by fine, uniform, rounded, radiating 

 strise, which increase by bifurcation, and give the surface an appearance 



