212 UNITED STATES GEOLOGICAL SURVEY OE TUE TEEEITOEIBS. 



upon the cardinal margin of a left valve, watching carefully at frequent inter- 

 vals for the appearance of the teeth. At first, a delicate ^.-shaped tooth made 

 its appearance under the anterior side of the point of the beak, directed 

 obliquely forward ; then, after a little farther grinding, a slender, anterior, 

 lateral tooth was seen with its posterior end close over the front division 

 of the A_-*haped cardinal, and extending forward close under the cardinal 

 margin, but with a marginal furrow above it for the recej^tion of a similar 

 anterior lateral tooth in the other valve. Behind the ^-shaped cardinal tooth, 

 there also appeared, near the cardinal margin, a thin lamina or tooth, ranging 

 very ohliquely backward, so as to leave a wide space between it and the 

 ^-shaped cardinal, evidently for the reception of a cartilage-pit; the bottom 

 of which, however, being hidden by the rock, could not be developed without 

 grinding away all of the teeth. Exactly whether the oblique thin tooth or 

 lamina behind, and extending forward over the cartilage-pit, is merely the 

 raised margin of the latter, as in Cymbophora, or the anterior end of an elon- 

 gated posterior lateral tooth, could not be determined ; but there is evidently 

 above and behind it, a long, narrow, marginal furrow for the reception of a 

 posterior lateral tooth in the right valve. 



From these hinge-characters, it is quite evident that this shell, notwith- 

 standing its Tellina or Arcopagia-Uke look, belongs to the Mactridce. It, 

 however, is quite unlike any of the described genera of the Machines, in form, 

 and more nearly resembles some types of the Lutrariince, such as Heterocardi a, 

 to which I have been somewhat inclined to refer it. 



The objection to referring it to the latter genus, however, is the fact 

 that its valves seem to be closed all around, while those of Heterocardia are 

 gaping behind. For the present, I have concluded to refer it provisionally 

 to the genus Mactra, rather as a much nearer approximation to its true posi- 

 tion, than as an expression of the opinion that it really belongs to that genus. 

 In form it agrees, however, rather more nearly with Spisula, and it is possible 

 that I would be nearer right to call it Spisula nitidula. Until a more satis- 

 factory view of its hinge can be had, however, its true position must remain 

 doubtful. 



Locality and position. — Deer Creek, near the North Branch of Platte 

 River ; from the upper part of the Fort Pierre group, or the lower beds of 

 the Fox Hills group of the Upper Missouri Cretaceous series. 



