208 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES 



Mactra (Cyinboplioral) Warrcnana, M. & H. 



Plate 30, figs. 7, a, b, e, d. 



Mactra Warrcnana, Meek and Hayden (185G), Proceed. Acad. Nat. Sci. Philad., VIII, 271. 

 Compare Alaclra (Cyinboplwra) formosa, M. & H., pi. 31), fig. 7. 



Shell rather thin, subtrigohal, moderately gibbous; cardinal border 

 sloping from the beaks in front and rear, at an angle of about 110° ; poste- 

 rior slope slightly more convex than the anterior ; extremities subangular, 

 the anal end being a little more obtuse than the other; base forming a broad, 

 regular curve ; beaks moderately elevated, incurved, and approximate, rather. 

 gibbous and nearly central ; lunule lance-oval, large, faintly impressed at the 

 margins, and extending from the beaks nearly or quite to the anterior 

 extremity ; escutcheon having the same form, but still larger, and bounded on 

 each side by a faint ridge, which passes from the beaks to the lower part of 

 the anal border, to the outline of which it imparts a very slight angularity. 

 Surface marked by faint, irregular lines of growth, which become more regular 

 and distinct on the lunule and escutcheon. Muscular impressions not very 

 deep, the anterior one narrow-ovate, pointed above, and a little arcuate, the 

 other ovate ; sinus of the pallial impression moderate, and rounded at the 

 extremity. 



We have not yet seen the hinge of this species, but its form and other 

 characters are such as to leave little room for doubt respecting its generic 

 relations. It is related to our 31. Siouxensis, but its anterior margin is more 

 angular in outline, and its pallial sinus is narrower and less broadly rounded 

 at the extremity.* These differences, and their widely different geological 

 positions, leave little reason to question the propriety of separating these 

 shells specifically. 



It is also very similar to our 31. formosa, from which I am even led, by 

 later comparisons, to suspect that it may not be distinct, though it seems to 

 lie more gibbous, and to have more prominent and ventricose beaks, while its 

 dorsal slope, just in front of the beaks, is more concave in outline. 



The specific name was given in honor of Lieut, (now General) Gr. K. 

 Warren, of the United States Topographical Engineers. 



Locality and position. — Moreau Kiver and Long Lake, Dakota ; in the 

 Fox Hills group, or No. 5 of the Northwestern Cretaceous series. It also 

 occurs at a somewhat higher position near Greely, Colorado. 



* The pallial sinus is represented a little too wide and rounded in our fig. 7, d, which also makes 

 tlic anterior outline of the shell too round. 



