THE MOLLUSCA OF COLORADO 89 



6. Umbilicus wide, exhibiting all volutions from below, striae riblike, width 

 5 to 5 . 75 mm., height 2 . 5 to 2 . 75 mm. 



Pyramidula cronkhitei anthonyi 



7. Shaped as anthonyi, but smoother, irregulariy wrinkled or ribbed. 



Pyramidula cockerelli 



8. Minute, reddish or brownish, marked by strong striae and fine spiral 

 lines, strongest on base, width i mm., height, o . 5 mm. 



Punctum pygmseum 



bb. Elevated, smooth, shining, amber-colored, whorls 6, increasing regularly, 

 umbilicus not perforated or scarcely so, width 2 .25 to 3 mm., height i .75 

 to 2 . 75, juveniles easily mistaken for young Pupa. 



Euconulus trochifonnis 



3. Minute, umbilicate, depressed, apex raised above top of last whorl, persistome 



everted or reflected, light unicolored, width 2 to 2 .6 mm., height i to i .3 mm. 



aa. Rather solid, shining, not ribbed, growth striae fine, persistome reflected 

 and white. Vallonia pulchella 



bb. Solid, striae stronger and more distinct than in pulchella and rather irregular, 

 last whorl expanding and descending toward aperture, peristome reflected 

 and porcelain-white. Vallonia gracilicosta 



cc. Thiuj ribs fine and numerous and bearing membranes which give the ribs of 

 fresh specimens a roughly sharp appearance, last whorl expanding and 

 descending toward aperture, persistome everted but not reflected into a 

 decided Up. Vallonia cyclophorella 



B.^ Height of shell greater than width. 



a. Aperture very large, peristome simple, shell thin and fragile, imperforate, whorls 

 usually 3, increasing rapidly. 

 I. Long and narrow. 



aa. Aperture long and narrow, exhibiting all volutions within to apex, last 

 whorl more than twice size of others combined, pellucid, color light or 

 greenish horn, growth lines minute, height 15 to 20 mm., width 6.5 to 9 mm. 



Succinea retusa 



bb. Aperture but little more than half length of shell, more nearly circular than 

 in retusa, color straw or greenish horn, sometimes with a reddish tinge, 

 shining, whorls 3, height 6 to 12 mm., spire sharp, growth lines minute 

 (our specimens all small, about 5 mm). Succinea avara 



cc. Pale horn or whitish, spire short and acute, coarsely striate, shining, height 

 20 mm., width 8 mm., aperture 13 mm., long and widest below middle. 



Succinea sillimani 



dd. Pale greenish horn-color, striae coarse and irregular, surface without liister, 

 aperture widest about middle and f the length of shell, height of shell 

 12 mm., width 6 mm., fold of columella distinct. Succinea rusticana 



ee. Color dull horn, growth hues rude, aperture about § length of shell, last 



