THE NATURAL HISTORY SURVEY. 181 



distant; color brownish-horn, transparent; periphery rounded; 

 sutures well impressed; whorls four, rapidly enlarging, the last 

 half of the last whorl very rapidly enlarging as it approaches 

 the aperture; spire flat or a trifle convex; aperture large, trans- 

 versely rounded, somewhat expanded; peristome simple, with 

 thickened edge; umbilicus small, round, deep, showing several 

 of. the volutions, the umbilical region strongly concave; base 

 of shell convex (See Fig. 36). 



Greater diam.,4.25; lesser, 3.75; ht.,2.50; umbilicus diam., 0.50 mill. (10103.) 

 " 4.00; " 3.50; " 2.25; " " 0.50 " (10104.) 



" 3.75; " 3.00; " 2.00; " " 0.50 " (10652.) 



" 4.50; " 4.00; " 2.50; " " 0.75 " (12290.) 



Animal: With a long, narrow, transparent foot, with pedal 

 grooves as in arboreus, head, neck and upper part of animal 

 black or blackish, lighter on the side of the foot; the other 

 characters as in arboreus. Heart situated as in the previous 

 species; pulsations regular and rapid, 95 per minute when con- 

 tracted and 1 30 when extended. Length of foot 6.00, width 

 i.oo mill. (Shell 4.25 mill, greater diameter.) 



Jaw: Strongly arched, wide, roundly pointed at the ends, 



FIG. 37. 

 Jaw of VITREA HAMMONIS Strom. (After Morse.) 



which are a little upturned; concave margin with a good sized, 

 rounded median projection, and two or three smaller projec- 

 tions on either side (Fig. 37). 



Radula formula: Y+f+i+f+ (27-1-27); teeth of 

 the same general type as those of Zonitoides arboreus ; central 

 narrow, tricuspid; laterals of same type, but bifid; marginals 

 aculeate. There are about 55 rows of teeth. 



Distribution: North America, Europe, Asia. In North 

 America from British America and Northern United States 

 south to South Carolina and Colorado. 



Geological distribution : Pleistocene; Loess. 



Habitat: Same as Zonitoides arboreus and usually asso- 

 ciated with that species. 



Remarks: This species is very closely related to Zonitoides 

 arboreus but may be distinguished by its darker color, the more 

 rapid enlargement of the last whorl and its different shape 



