CALL AND PILSBRY ON PYRGULOPSIS. 13 



This species has a limited distribution, occurring only, so far as 

 present information goes, in North-western Nevada, in Walker and 

 Pyramid Lakes. Living forms have been collected only in the last- 

 named locality. 



The description of Dr. Stearns has been amended to form a diag- 

 nosis as complete as very extensive series would permit. The shell is 

 exceedingly variable, ranging from short and stout to long and slender, 

 and from strongly carinate to entirely smooth. Figures 6, 7, and 8 of 

 Plate II. show the smooth forms and the variations to which they are 

 inclined. 



Pyrgulopsis mississippiensis, sp. nov. 



(Plate II., Fig-s. 14-16.) 



Pyrgula scalar if ormis, var. mississippiensis Pilsbry. Am. Nat., Jan. 1886, p. 

 75. No description. 



Shell minute, pupiform, elevated, imperforate; whorls 5/^-6, flat- 

 tened, with a well-marked carina on lower third, which becomes cen- 

 tral on periphery of last whorl, body-whorl equal to one-half entire 

 length of shell and bluntly angulated at position of carina; epidermis 

 ; suture distinct, deeply impressed only at commencement of 

 last whorl and below, above last whorl covered by carina; aperture 

 narrowly ovate, broadly rounded before and narrowly rounded behind, 

 very oblique; peritreme continuous, almost rimate, slightly reflected 

 over the parietal wall ; lip simple, sharp. 



Animal unknown. 



This form has hitherto occurred only in the Mississippi River, near 

 the mouth of Rock River, on the Illinois side Rock Island County, 

 Illinois. Many dead specimens have been taken, but, as yet, no living 

 ones. It differs in important particulars from the P. scalar if or mis, 

 Wolf, which is "carinate its entire length," has a different aperture, 

 and is markedly different in the character of the sutures. Our species 

 is ecarinate on all whorls above the body-whorl, the carina being de- 

 pressed and filling entirely the suture. The outlines of the apical 

 whorls are wholly unlike the figured type of Wolf's species. The aver- 

 age measurements of the four specimens upon which the preceding 

 description is based' are, for length, 4.66 mm ; for breadth, i.6i inm . The 

 proportions of length and breadth, while variable, do not vary within 

 so wide limits as the P. nevadensis. The individual measurements are 

 as follows: 



Specimen i. Length, 3.58"; breadth, i.6o mm . 

 2. 3-38 mm ; 1.56". 



j. 3-5o mm ; i. 64". 



4. " 3.40"""; " 1.64""". 



