42 GASTEROPODA. 



Laud-snails and Pond-snails tliere is a well-developed shell 

 into which the animal can retire completely. Tlie Slugs, 

 again, have a merely rudimentary shell which is completely 

 concealed within the mantle. The completely shell-less forms 



Fig. 432. — Eccnliompludus distans. Quebec Group (Lower Silurian). 



are necessarily wholly unknown as fossils. The Slugs, with 

 a rudimentary shell, are only doubtfully represented in a 

 fossil state, and that only in the Tertiary rocks. The abun- 

 dance of the shell-bearing forms as fossils depends mainly 

 on the liabits of the animal. The Land-snails, being terres- 

 trial in their habits, are, necessarily, but sparingly represented 

 as fossils, and they do not date back to a time anterior to 

 the Carboniferous. The Pond-snails, being exclusively con- 

 lined to fresh water, are only known as fossils in tluviatile 

 and lacustrine deposits, and they are exclusively Secondary 

 and Tertiary, not being known in the Paheozoic period. 

 The F'ldmonifera are divided into the two orders of the 

 Inoperculata and Operculata, according as the shell is des- 

 titute of an operculum, or is provided with this apparatus. 



Order IV. Inoperculata. — ^Jidl not p)^'ovidcd tvitli cm 

 (I'percuhim. 



Fam. 1. HeltciD/E. — Shell well developed, capable of con- 

 taining the entire animal. With the exception of Pupa, 

 Dawsondla, and Zonitcs (the last a sulj-genus of Helix), all 

 the Hdicidm belong to the Tertiary and Eecent periods. As 

 they are all terrestrial in their habits, they are necessarily 



