﻿28 Bulletin 20 106 



Pedicle valve uniformly and strongly convex, without sinus. 

 Beak projects slightly beyond the opposite valve. Line of 

 junction of the two valves bends upward gently against the 

 dorsal valve at the front, suggesting a sinus. 



Brachial valve hemispheric without fold. 



Surface marked by fine concentric striae and stronger distinct 

 lines of growth. Very weak plications are present at the anterior 

 margins of the valves, but these are unrecognizable in the middle 

 and posterior portions of the shell. 



Shell impunctate. 



The species is represented by a single specimen which measures 

 5^ mm. in length and 5mm. in breadth. 



The absence of a fold and sinus distinguishes it from the other 

 species of the genus to which it is provisionally referred. 



Locality. — Princeton, Montana. 



Atrypa reticularis (Linn) 

 PI. I. fig. 6. 

 Typical examples of this species occur in this fauna. It is not, 

 however, a common species and it has not been observed in 

 association with any of the other species of the genus. 



Locality. —Sixteen P. O. and Flat Iron Mountain, Lewis and 

 Clark Forest Reserve, Montana; Manson station, Idaho. 



Atrypa spinosa var. montanensis n. var. 

 PI. I figs. 5--5C 



Shell small, suborbicular, valves equally and moderately 

 convex. 



Ventral valve with pointed beak extending beyond the oppo- 

 site valve but not incurved. 



Dorsal valve with small, closely incurved beak. 



Surface of each valve marked by 14 to 20 strong rounded 

 costae bifurcating at irregular intervals and increasing in strength 

 from the umbonal region to the margin. These are crossed by 

 lamellae which form concentric bands of nodes on the costae. 

 Both valves are regularly convex except in the portions near the 

 outer extremities of the hinge line, which are somewhat flattened. 



