206 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



and pallial impressions of this genus, that we can hardly be far wrong in 

 referring them to it. In the Tertiary rocks, we also find this genus well 

 represented, as well as in the existing seas. 



The living species are widely distributed, and usually occur along sandy 

 shores, buried a little beneath the surface. 



Mactra (Cynibopliora!) S ion x ens is, M. & H. 



Plate 1, figs. 7, a, b, c. 

 Mactra Siouxensis, Meek and Hayden (May, 1860), Proceed. Acad. Nat. Sci. Pkilad., VIII, 179. 



Internal cast oval-subtrigonal, moderately gibbous ; anterior border 

 narrowly rounded, posterior subangular at the extremity ; base forming a 

 nearly semi-ovate curve, the most convex part being toward the front ; dorsum 

 declining with a slightly convex outline behind the beaks, and distinctly 

 concave just in front of them ; beaks prominent, rather gibbous, very nearly 

 central ; pallial impression provided with an oval sinus, which appears to 

 be a little narrower behind than in the middle, rounded at the anterior 

 extremity, and extending nearly in a horizontal direction forward, about one- 

 fourth of the length of the valves. 



Length, 1.55 inches; height, 1.22 inches; convexity, 0.76 inch. 



The only specimens of this species that I have seen are internal casts 

 and moulds of the exterior, the shell itself being dissolved out. Some of 

 these moulds show that the surface was marked by moderately distinct lines 

 of growth, and that the escutcheon was lanceolate in form, and bounded on 

 each side by a very obscure ridge, which extends from the back part of the 

 beaks to near the postero-basal extremity. 



Although related to our M. Warrenana this shell is more ventricose, has 

 more elevated beaks, and its anterior border is less angular in outline, while 

 the dorsum just in front of the beaks is much more concave. 



Locality and position. — On Big Sioux Eiver, two miles above its mouth, 

 in a sandstone of the Dakota group, or formation No. 1 of the Cretaceous 

 series of the Upper Missouri. 



