1856.] 110 



three times as large, aud differs in baviag stronger and more elevated linea of 

 growth. It may be an elevated Helix. 

 Loc dity. Three miles above Fort Union. 



LlMN^A TENUICOSTA. 



Shell small, turrited, very slender ; spire acutely elevated ; volutions four to 

 four and a half, vertically flattened, (or slightly convex,) increasing rapidly in 

 the direction of the longitudinal axis of the shell, and each so much smaller 

 than the succeeding one below it, as to form a shoulder or offset at the suture ; 

 surface ornamented by sharply elevated, equidistant, linear folds parallel to the 

 minute lines of growth ; suture very oblique, rather strongly impressed ; aper- 

 ture unkiown. Length -29 inch; breadth -08 inch ; apical angle regular, di- 

 vergence about 26. 



This exceedingly slender, delicate little Limnea, is not apt to be confounded 

 with any other fossil or recent species with which we are acquainted. Its at- 

 tenuated form, vertically flattened whorls, and peculiar linear folds, are charac- 

 ters by which it will be easily identified. None of our specimens show the 

 aperture, though it must be narrow, and a little shorter than the spire. It is ii 

 very rare shell. 



Locality. Three miles below Fort Union. 



PhYSA liONGIUSCULA. 



Shell elongate ovate ; spire slender, rather elevated, acute at the apex; whorls 

 about six, flattened or slightly convex ; suture not very distinct ; surface marked 

 with fine, straight, nearly obsolete lines of growth, which cross the whorls at 

 right angles to the suture ; aperture apparently narrow ; outer lip meeting the 

 body whorl at an acute angle above. Length -59 inch ; breadth -27 inch ; apical 

 angle regular, divergence 43. 



Our specimens are too imperfect to show the exact form of the aperture, 

 though it appears to have been narrow, very acutely angular above, and nar- 

 rowly rounded below. The lines of growth are faint, and the substauce of tiie 

 shell thin. This species quite closely resembles some varieties of P. hypnorun 

 (Lin.,) but the body volution is relatively wider, the spire somewhat less ele- 

 vated, and the aperture narrower. 



Locality. Three miles above Fort Union. 



Physa rhomboidea. 



Shell small, oval, narrowing abruptly from the middle towards both extremi- 

 ties ; spire rather short, conical, pointed ; volutions four and a half, slightly con- 

 vex, last one large, but scarcely ventricose ; surface marked with fine lines of 

 growth ; suture distinct ; aperture narrow, acutely angular above, narrowly 

 rounded below ; inner lip closely spread upon the body whorl above, and forrti- 

 ing a fold below the deeply impressed umbilical region. Length -26 inch ; 

 breadth -10 inch ; length of aperture -15 inch, breadth of do. -07 inch ; apical 

 angle nearly regular, divergence 57"' 



The smaller size, less elongated form and shorter spire, will serve to distiii- 

 guish this species from the last. From the same bed we have a few other specv 

 mens, which have a relatively wider and more oblique aperture, and a more 

 ventricose body whorl, but they are so nearly identical in other respects with 

 this, that we can only regard them as a variety of the same. 



Locality and position. Same as last. 



Physa Nebrascensis. 



Shell large, ovate, thin ; spire rather short; whorls about five, flattened or 

 slightly convex ; suture very oblique, not strongly impressed ; surface marked 

 with coarse lines of growth ; aperture narrow, acutely angular above, narrowly 

 rounded below ; columella impressed in the umbilical region, and having a kimd 



