GEOLOGICAL SURVEY OP THE TERRITORIES. 305 



curve; posterior margin subtruncated or rounded; anterior margin 

 rather more narrowly rounded; dorsal outline sloping rather abruptly 

 and nearly equally, with slight convexity in front and rear of beaks, the 

 anterior slope being somewhat more abrupt than the other ; beaks nearly 

 or quite central, curved inward and slightly forward ; posterior dorsal 

 surfaces a little flattened ; post-umbonal slopes somewhat prominent, or 

 sometimes almost subangular in internal casts; muscular impressions 

 shallow and oval ; pallial line with a small obtuse sinus. External sur- 

 face unknown ; surface of internal casts smooth. 



Length, 0.47 inch ; height, 0.42 inch ; convexity, 0.26 inch. 



Impressions of the hinge of this little shell, left in the matrix, show 

 that it has elongated anterior and posterior lateral teeth that are striated 

 transversely, as in Corbicula. In the left valve there, is one elongated 

 anterior, and one similar posterior lateral tooth, each of which fits into 

 a corresponding furrow in the margin of the other valve. Immediately 

 above this furrow, in the posterior and anterior margin of the right valve, 

 there is a smaller linear, striated lateral tooth, while the margin below 

 the furrow seems hardly to project enough to be properly regarded as a 

 second anterior and posterior lateral tooth. The specimens of the im- 

 pressions of the cardinal teeth are not so clearly seen, being mainly 

 hidden behind the cast of the umbones, in the only specimen that I have 

 seen with this part of the hinge well preserved. So far as they can be 

 made out, they seem to be much as in the existing species of Corbicula, 

 excepting that the anterior cardinal tooth of the left valve is directed 

 almost horizontally forward instead of obliquely forward and downward; 

 it is also much compressed from above and below, and very prominent, 

 and curved upward. The corresponding tooth of the other valve is 

 much smaller and overlaps that of the right valve. Behind this promi- 

 nent anterior cardinal tooth of the left valve, I hink I have seen two 

 other diverging and emarginate cardinal teeth, with pits for two corres- 

 ponding diverging teeth in the right valve. The number and arrange- 

 ment of the teeth of the hinge would, therefore, if correctly understood 

 in the specimen, seem to be almost exactly as in Corbicula, with the 

 exception of the horizontal direction of the anterior cardinal teeth, and 

 the prominence of that of the left valve. 



It is quite possible, however, that if we had specimens showing more 

 clearly the cardinal teeth, we might find differences enough to warrant 

 the establishment of a distinct section, allied to Corbicula. 



Locality and position : Twelve miles southwest of Salina, Kansas; 

 Dakota Group of the Upper Missouri cretaceous series. Professor 

 Mudge. 



CORBICULA? SUBTRIGONALIS, (MEEK.) 



Shell oval-subtrigonal, about one-fourth longer than wide, rather 

 gibbous, the greatest convexity being above the middle; basal outline 

 forming a semi-elliptic curve; extremities rather narrowly and very 

 nearly equally rounded; beaks somewhat depressed and very nearly 

 central; dorsal outline sloping before and behind the beaks, the latter 

 slope being convex and the former nearly straight. Surface only show- 

 ing five lines of growth. Pallial line with a small, obtusely-subangular 

 sinus. 



Length, 1.16 inches; height, 0.90 inch; convexity, about 0.66 inch. 



The only cast of the hinge of this species I have seen, is that of a left 



valve. It shows the impressions of the same elongated and striated 



lateral teeth seen in the last. The cardinal teeth, however, seem to 



have unfortunately been much injured by maceration or erosion, before 



20 G 



