1906.] NATURAL SCIENCES OF PHILADELPHIA. 137 



While only weakly ciiaracterized, this form seems to range over a 

 considerable area, and apparently deserves recognition by name. 

 B. dealbatus ragsdalei (Pils ). PI. VI, flgs. 16, 17. 



Nautilus, III, p. 122; Man. of Conch., XI, p. 129. 



The shell varies from the ovate shape of typical dealbatus to a more 

 lengthened and slender form, and is conspicuously rib-striate, the striae 

 white on a tawny or white-blotched ground and weaker on the base of 

 the shell. The lip-rib is strongly developed. Three adult specimens 

 of the type lot measure: 



Only dead, more or less bleached shells have been taken from the 

 top of the Red river bluff at the southern end of Warren's Bend, 

 twenty-five miles northwest of Gainesville, Cooke county, and a mile 

 north of St. Jo, Montague county, Texas. 



Across the whole State of Texas nearly 400 miles distant, this form 

 reappears on the Rio Grande river, in Val Verde county, in an area 

 inhabited also by B. alternatus marice. We are quite unable to find any 

 constant differences between these Southwestern shells and the Red 

 river types, although the apparent absence of the form in the inter- 

 vening territory suggests that the similar forms of the two areas are 

 independent parallel modifications of dealbatus stocks, rather than 

 actually connected genetically. Yet it is quite possible that the 

 widely sundered colonies have been or still are connected through 

 the great conchologically unknown area northwest of the oblique line 

 across the State marking the limit of our explorations. Dead, bleached 

 shells were found in abundance on the high land west of Devil's 

 river, but the living ones for some time eluded us. Finally we found 

 them hidden under the dead reversed leaves which thatch the trunks 

 of Yuccas, and sometimes under prostrate dead Yuccas — retreats they 

 share with the smooth race of Polygyra texasiana (pi. VI, figs. 18 to 

 22). The proportions vary a good deal, a series of adult shells 

 measuring : 



17 mm. 



6^ 



