1856.] 117 



CORBULA MACTRIPORMIS. 



Shell subtriangular, scarcely ventricose ; right valve a little larger, thicker 

 and more gibbous than the left ; extremities narrowly rounded ; posterior end 

 longer than the anterior, slightly truncate at the extremity, and having an ob- 

 tuse ridge passing from the beaks obliquely backwards to the lower posterior 

 edge ; base semi-ovate, most prominent in advance of the middle ; beaks con- 

 siderably elevated, pointed, incurved, and directed forward ; surface marked with 

 fine line's of growth. Length -64 inch ; breadth about -33 inch ; height -50 inch. 



The tooth of the right valve is thick, and located immediately under the 

 beaks, while that of the left is flattened, and placed a little behind them. 

 The lateral edges of the cardinal border of the left valve, as well as its basal 

 margin, which are sharp and prominent, fit into a distinct groove in the edge of 

 the opposite valve. The muscular im jressions are indistinct, and the sinus of 

 the palleal impression triangular, very broad and shallow. 



Locality. Fort Clark, where it is associated with Melania, Paludina, &c. 



Unio priscus. 



Shell ovate, rather compressed, very thin and fragile ; anterior extremity 

 short, rounded ; posterior end narrower, contracting with a regular curve from 

 above, and having at the extremity below a very obtusely rounded angle ; car- 

 dinal border broadly arcuate ; basal margin nearly straight behind the middle, 

 rounding up in front ; beaks very small, rising little above the hinge, located 

 about one-sixth the entire length of the shell behind the front, and ornamented 

 with small regular concentric wrinkles ; surface of other portions of the shell 

 smooth, or only marked with fine lines of growth. Length 2-78 inches; breadth 

 unknown ; height 1-63 inches. 



Has much the aspect externally of an Anodonta, but a fragment in our collec- 

 tion shows enough of the hinge to prove it to be a Unto, without exhibiting the 

 details of the teeth. Judging from the beaks, young specimens not more than 

 half an inch in length must be beautifully ornamented with regular concentric 

 wrinkles. It is usually found in a crushed condition between the laminae of 

 clay. 



Locality. Yellowstone River, forty miles above the mouth. 



BULIMUS ? TERES. 



Shell small, sinistral, much elongated, terete ; volutions ten to ten and a half, 

 narrow, closely wound, and increasing very gradually from the apex, slightly 

 convex near the summit of the spire, but flattened lower down ; suture very 

 faintly impressed between the lower volutions, but becoming more distinct to- 

 wards the apex ; surface marked with fine, regular lines of growth, passing 

 straight across the whorls at right angles to the suture ; aperture ovate, acutely 

 angular above, rounded below ; lip thin. Length -76 inch; breadth -19 inch; 

 apical angle slightly convex, divergence 18. 



A few dextral shells were found associated with the above, which, as far as 

 we have been able to see, diifer from them in no other respect. It is possible, 

 however, they may belong to a distinct species, though we are now inclined to 

 regard them as merely a variety of the same. This and the following species 

 are evidently closely related to a shell described by M. Matheron, from the Ter- 

 tiary lignites near the mouth of the Rhone, south-east of France. (See Melania 

 adcula, p, 219, pi. 36, fig. 25 ; Catalogue Methodique, &c.) Like some of ours, his 

 is a sinistral shell, and has much the same general appearance, but it is even 

 more elongated, and has nearly twice as many volutions. We doubt very mnch 

 the propriety of referring such forms to the genus Melania., as they appear to us 

 to have much more the aspect of land shells. Unfortunately all our specimens 

 have the aperture more or less broken or distorted, as was the case with those 

 studied by M. Matheron. The spire looks very like Clausilia, but the aperture 

 was evidently more like Bulimus or Achatina. It is not improbable they may 



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