ILLINOIS NATURAL HISTORY SURVEY MANUAL 



denticles^ folds or lamellae^ sometimes improperly called teeth. 

 The surface of the shell exhibits a feature called sculpture^ 

 represented by growth lines. These are made as the animal 

 adds to the size of its shell by depositing a limy substance at the 

 edge of the aperture from the little cells bordering its soft parts. 

 The sculpture may be very fine, like thread wound tightly on a 



Apex 

 Suture 



Labial Denticle 

 Peristome 

 Peristomal Denticle 



Umbilicus 



Parietal Denticle 

 Parietal Wall 

 Labial Denticle 

 Aperture 



Fig. 8. — Named parts of shell of the snail Polygyra tridentata Jrisoni 

 F. C. Baker. 



spool, or it may be so coarse that it forms elevations called ribs. 

 Sometimes well-defined spiral marks or lines are added, forming 

 in many cases a latticed pattern. 



Color of shells. — Varying widely in color and pattern, the 

 shells of the land snails of lUinois occur in many different shades 

 of yellow, brown and gray. Some are of uniform color; others 

 are marked by one or more spiral bands or a number of zig- 

 zag spots and cloudings of darker color. Shells of many of the 

 forms, for instance some of the pupoids, are translucent and like 

 paraflin in appearance; other shells are opaque and of a dark 

 brown or nearly black color. Certain species have a glassy or 

 shiny shell of a brownish color. 



All of the variations in size, form and color of shell enter 

 into the naming of snails. 



Naming snails. — Snails are known by their scientific names, 

 which are always of Latin form and are often of Greek or Latin 



