1856.] 271 



tremities, he would have omitted to mention it. It is more narrowly rounded 

 in front than any of the casts we have seen from the New Jersey formations, or 

 than any species we have seen figured in foreign works. 



Locality and position. Sandstone near mouth of Judith River, probably No. 1 

 of the section. 



MaCTRA FORMOSA. 



Shell thin, triangular oval, somewhat compressed ; cardinal border sloping 

 from the beaks at an angle of about 115, straight in front, and slightly convex 

 behind the beaks ; extremities narrowly rounded, almost angular, nearly alike ; 

 base forming a broad regular curve ; beaks small, elevated, approximate, central 

 or a little in advance of the middle ; surface marked with fine lines of growth, 

 which become stronger and more regular on the large lanceolate lunule and 

 escutcheon. Length I'TO inches ; breadth '82 inch; height 1-36 inches. 



The lunule and escutcheon extend from the beaks to the extremities of the 

 shell, the former being slightly impressed, and the latter bounded by a very 

 obscure ridge. By grinding down upon the cardinal edge of a right hand valve, 

 we found it thin, and having under the beaks apparently an oblique cardinal 

 tooth and a small ligamentary pit, the former being divided into two diverging 

 parts ; while the anterior and posterior portions of the edge have each a long 

 groove, probably for the reception of slender lateral teeth in the other valve. 



Locality and position. Same as last. 



Mactra Warrenana. 



Shell triangular oval, moderately compressed ; cardinal border sloping from 

 the beaks at an angle of about 110 ; anterior end narrowly rounded ; posterior 

 end wider, rounded or very slightly truncate, and gaping a little at the ex- 

 tremity ; base forming a broad curve ; beaks elevated, rather small, incurved, 

 nearly central ; surface marked with fine lines of growth, which become stronger 

 and more regular on the large lanceolate lunule and escutcheon. Length 1-55 

 inch ; height 1-20 inch ; breadth -85 inch. 



The muscular impressions are oval or ovate; the anterior one being rounded 

 below, and contracted and prolonged above. The sinus of the palleal impres- 

 sion is small, obtuse at the extremity and nearly horizontal. 



This species is nearly related in most of its characters to the last, but may be 

 distinguished by its less compressed form, more elevated and gibbous beaks 

 and thicker shell. We have had no opportunity of comparing their internal 

 characters, nor have we seen the hinge of this species. Their distant geo- 

 graphical, and widely separated stratigraphic positions, in addition to the above 

 named differences, lead us to regard them as distinct. 



We name this species in honor of Lieut. G. K. Warren, of the U. S. Topo- 

 graphical Engineers. 



Locality and position. Yanktonin trading post, No. 5 of the accompanying 

 section. 



Mactra alta. 



Shell thin, subtriangular, compressed ; cardinal border sloping from the beaks 

 at an angle of about 80" ; base forming a broad regular curve ; extremities nar- 

 rowly rounded or subangular, and nearly alike ; posterior slope having an ob- 

 scure ridge or angle passing obliquely backwards and downwards from the beaks ; 

 umbones much elevated, (pointed ?) nearly central ; surface marked with con- 

 centric striae. Length about 2-62 inches; height 2-16 inches; breadth 1-15 

 inch. 



We have not yet had an opportunity of seeing the interior of this shell, nor the 

 details of its hinge, and all our specimens are worn so as to obscure the surface 

 markings. It resembles the last two species, but is larger and much more 

 compressed than either of them, and the much greater elevation of its beaks 

 gives it more nearly the form of an equilateral triangle. 



