18 UNITED STATES GEOLOGICAL SURVEY OE THE TERRITORIES. 



Variety A, pi. 10, figs. 3, e,/ — Obliquely broad oval; beak of under 

 valve considerably distorted, and the area nearly obliterated by the scar of 

 attachment; anal side having a broad shallow sinus near the binge, produced 

 or prominent near the other extremity ; anterior and antero-ventral border 

 forming a broad, nearly semicircular curve. 



Length and breadth, each about 3 inches ; concavity of under valve, 

 0.70 inch. 



Variety B, pi. 11, figs. 3, a, b. — Obliquely ovate, narrow near the beaks, 

 and widening toward the other extremity ; beak of lower valve somewhat 

 distorted on the left side by the scar of attachment, curving but slightly 

 upward ; lateral margin near the right side of the beak thickened, but thin 

 and contracted on the other side. 



Length, 3.10 inches: breadth, 2.30 inches; concavity of under valve, 

 0.35 inch. 



O. intermedia would be a good name for this form if further comparison 

 should show it to be a distinct species. 



Variety C, pi. 11, figs. 4, a, b. — Irregularly subovate, oblique, narrow 

 near the beaks, and abruptly widened by an expansion of the left border 

 near the other extremity ; umbo of under valve more or less distorted and 

 curving slightly upward ; border on right side of beak in same often thick- 

 enend, thinner and profoundly sinuous on the other side; area of upper valve 

 inclined obliquely toward the cardinal extremity. 



Length, 3.20 inches; greatest breadth, 2.37 inches; breadth near beaks, 

 1.20 inches. 



This differs so extremely from the other forms that it will perhaps be 

 difficult to convince those who have not seen all the intermediate gradations 

 that it is even remotely related to the form taken as the type of O. patina. 

 Indeed, the differences are so great that, had we only seen the two extremes, 

 I would not hesitate to regard the normal forms of O. patina as belonging to 

 the genus Gryphcea, and that now before me as a true oyster. Should the 

 latter be considered entitled to rank as a species, it may be designated as O. 

 subsinuata. 



Locality and position. — Two hundred miles above the mouth of Milk 

 River on the Missouri ; in Fort Pierre group, or formation No. 4 of the Cre- 

 taceous series. 



