BAKER: FIELDBOOK OF ILLINOIS LAND SNAILS 17 



than the width of the tray to permit a short cork to be placed 

 in the mouth. For a 2- by 1-inch tray, a bottle IH inches in 

 length is desirable. The larger shell specimens may lie loose in 

 the tray. 



It is well to index the collection either in a book or upon 

 library catalog cards. Much of the valuable information previous- 

 ly recorded on the field data blanks may be entered under the 

 name of each specimen. 



Cabinets are necessary, but they need not be expensive. 

 If available, a spool cabinet is very good. One of the best cabi- 

 nets for keeping a collection of land snails in good order is a 

 legal blank case, which is about 14 inches high, 10 inches wide 

 and 15 inches deep. It has 10 drawers, each drawer slightly less 

 than an inch deep and capable of holding several trays such as 

 have been described. These drawers are admirable for the land 

 snails found in Illinois. The case may be purchased from office 

 supply companies for about S6. 



With the case, the trays and the vials, the collector has 

 most of the equipment for beginning a valuable and interesting 

 collection of land snails. 



Classification of Land Snails 



In the study of land snails, a system of naming is used by 

 which they may be known to people interested in them. The 

 shell itself has certain features which are given names. By a 

 description of the form and position of these named parts of the 

 shell almost anyone may know or identify a snail. In fig. 8, 

 a shell of Polygyra tridentata Jrisoni is pictured with the parts 

 indicated. The shell in nature is covered with a horny envelope 

 called the periostracum or epidermis^ which protects the limy 

 part of the shell substance from destruction by the carbonic 

 acid in the air or water. When the snail perishes and its shell 

 dries, this epidermis usually peels off, exhibiting the chalky 

 nature of the shell substance. 



Form of shells. — The shells of Illinois land snails vary 

 greatly in form. They are rounded, elongated, flattened and disc- 

 like, or turreted. They range in size from that of one of the little 

 pupoids, less than one-sixteenth inch in length, to that of the 

 white-lipped snail, more than an inch in diameter. 



The aperture in different species varies in form and may be 

 modified by the presence of large or small projections known as 



