142 PROCEEDINGS OF THE ACADEMY OF [March, 



At Derby, Frio county (pi. VII, figs. 20, 21, 22, 23, 24), the shells are 

 bluish white with bluish streaks, a few brown tinted with brown 

 streaks. The columellar tooth is small or wanting. This form has 

 been called albidus Taylor, Ckll. 



At Laredo, Webb county (pi. VII, lower line of figs.), the shells are 

 large and mostly long. The lot consists of pm-e white, and of coffee- 

 tinted, brown-streaked shells, with all intermediate forms. A colu- 

 mellar tooth is exceptionally developed. 



At Del Rio, Val Verde county (pi. VII, figs. 9, 10, 11, 12), we found a 

 more conic i^ce. The shells are white, often with some very faint 

 grayish streaks. Interior varying from ochre-yellow to dark brown. 

 In the series of about sixty living shells before us, none has mottled 

 markings, and while in a few there is a weak indication of the colu- 

 mellar tooth, it is as a rule absent. The larger ones measure 29 to 33 

 mm. long, 16 wide. Some of the dead shells found in the same place 

 are larger, 35 x 18.5 to 37 x 17 mm. A single albino was taken alive 

 (fig. 12). These shells are from the plain along the San Fihpe river, 

 on the east side, not far from the Rio Grande. B. d. ragsdalei was 

 found in the same place. 



On the mesa west of Devil's river we found large dead shells like those 

 from Del Rio but even larger, often with a low lump on the parietal 

 wall, but no tooth on the columella. 39 x 20.5 to 35 x 16 mm. 



At the Pecos High Bridge, on agaves, north of the railroad near the 

 eastern end of the bridge, we found Bidimulus abundant and finely 

 developed (pi. VII, figs. 1 to 7). The shape varies, but is always more 

 conic than in the eastern localities. It is either nearly uniform white, 

 or coffee-tinted varied with oblique brown or purplish streaks; these 

 two color-forms in about equal numbers, found together on the same 

 plants and connected by intermediate examples. Apex white. The 

 interior is very dark purple-brown. The outer lip is usually but little 

 thickened within, and the columella has no tooth, though often it 

 is slightly salient in the middle. Whorls 7 to 7^. 

 Alt. 36 37.5 33.3 34.5 36 33.3 37 mm. 



Diam. 19 19 17 17 17 16 15 



Aperture 17.8 17.8 16 16 16 16 14.3 " 



The last two measurements show the extremes of shape in a series of 

 150 Hving specimens (No. 84,627 A. N. S.). 



Another set from east of the Pecos canyon about a mile from the 

 Rio Grande is similar. The largest one measures, alt. 38, diam. 20.3, 

 aperture 19 mm. (pi. VII, fig. 8). 



