88 UNIVERSITY OF COLORADO STUDIES 



2. Shell and hinge very slight, beaks in most species calyculate (with caps) . 



Calyculina spp. 

 (After consultation with our best authorities on these three genera it seems best to omit 

 any attempt to put the species in a key, in the present state of knowledge). 



GASTROPODA (Univalves) 

 * Terrestrial Snails, with Well-developed Shell 



A. Height of shell less than width. 



a. Width of adults more than 8 mm. 



1. Lip well reflexed. Polygyra monodon 



2. Lip not reflexed, whorls 5, marked by one or more dark spiral bands. 



aa. Depressed, sometimes reaching width of 25 mm. and height of 10 mm. 



Oreohelix strigosa 



bb. Smaller with same number of whorls, wddth 16 mm., height 8 mm., other- 

 wise as strigosa. Oreohelix strigosa concentrata 



cc. Spire elevated, last whorl descending toward aperture. 



Oreohelix strigosa cooperi 



dd. Similar to strigosa but with several riblike revolving lines crossing the 

 lines of growth. Oreohelix haydeni 



b. Width less than 8 mm. 



1. Discoidal, apex level with or below top of last whorl. 



aa. Whorls not equally visible above and below, growth lines not crossed by 

 parallel spiral lines, whorls 5 or 6, width 4 mm., height 2 . 5 mm. 



Thysanophora ingersolU 

 bb. Whorls equally visible above and below, growth lines crossed by fine, 

 equidistant, parallel spiral lines, width 3 . 5 mm., height i . 5 mm. 



Helicodiscus parallelus 

 cc. Larger, width 4.5 to 5 mm. H. eigenmanni arizonensis 



2. Apex raised more or less above top of last whorl, peristome not everted or 

 reflected. 



aa. Rather depressed. 



1. Thin, brownish horn, smooth, shining, whorls 4, last enlarging very 

 rapidly, width 3 . 5 to 4 mm. Vitrea hammonis 



2. Very fragile, greenish white, transparent, smooth, glassy, whorls 3, last 

 composing most of shell, width 5 mm. Vitrina alaskana 



3. Smooth, growth lines fine, requiring a lens, color amber to whitish, 

 whorls 4 or 5, width 5 mm., heighth 3 . 75 mm., usuallj' smaller. 



Zonitoides arboretis 



4. Minute, spire nearly flat, growth lines strong under lens, color white, 



width 2 . 5 mm., height o . 75 mm. Zonitoides minusculus 



5. As minusculus, except striae more ribUke, interspaces microscopically 



striate, spire less depressed. Zonitoides conspectus 



