INVERTEBRATE PALEONTOLOGY. 207 



M a c t r :i ( C y m l» o p li o v a ? ) formosa, M. & H. 



Plate 39, fig. 7. 

 Madia formosa, Meek ami Hayden (Nov., 1S5G), Proceed. Acad. Nat. Sci. Philad., VIII, 271. 



Shell very thin, transversely ovate-subtrigonal, somewhat compressed ; 

 cardinal border sloping from the beaks at an angle of about 115° ; posterior 

 slope a little more convex than the anterior, which is nearly straight or slightly 

 concave in outline above ; extremities subangular, and very similar, excepting 

 that the angle of the posterior is lower than the anterior ; base forming a 

 broad semi-oval or semi-elliptic curve ; beaks small, rather depressed, in- 

 curved, approximate, and located near the middle of the shell. Surface 

 marked by fine lines of growth, which become stronger, or rise into very 

 small regular wrinkles on the large lanceolate lunule and escutcheon. 



Length, 1.70 inches ; height, 1.36 inches ; breadth, 0.82 inch. 



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

 the shell ; the former being defined by a very slightly impressed margin, and 

 the latter bounded on each side by an obscure ridge, which passes from the 

 back part of the beaks to a point below the middle of the posterior margin, 

 to which it imparts a subangular outline. 



None of our specimens are sufficiently detached from the matrix to show 

 very clearly the internal characters ; but by grinding down upon the hinge of 

 a right valve, it was found to be quite thin, and provided under the beak with 

 a rather small, oblique, spoon- shaped cartilage-pit; while the posterior and 

 anterior margins have each a long, sharply-defined groove, for the reception 

 of the slender lateral teeth of the other valve. The cardinal tooth of this 

 valve is very short and close to the margin. The hinge of the other valve is 

 unknown. 



Locality and position. — Mouth of Judith River, Idaho Territory ; in beds 

 holding a position at the horizon of the top of the Fox Hills group. 



