PULMOGASTROPODA. 8ll 



developed external shell, within which the animal can entirely with- 

 draw itself. 



As regards their distribution in time, the completely shell-less 

 forms are necessarily altogether unknown in the fossil condition, 

 and the forms with a rudimentary and concealed shell are only 

 known in the latest Tertiary deposits. The abundance of the or- 

 dinary shell-bearing forms as fossils depends mainly on the habits of 

 the animal. As the preservation of an ancient land-surface in the 

 crust of the earth is a matter of rare occurrence, the strictly ter- 

 restrial Pulmonates are not largely represented in the fossil state, 

 their remains, in fact, occurring principally in lacustrine or fluviatile 

 deposits intermingled with the shells of fresh-water types of Pul- 

 monates. These latter are found in moderate numbers in fresh- 

 water deposits of Secondary and Tertiary age. The oldest known 

 types of the Pulmonate Gastropods have been found in the Carbon- 

 iferous rocks. 



The sub-class Pulmogastropoda may be divided into the two orders 

 of the Stylommatophora and the Basommatophora ; but only a few of 

 the more important families composing these can be noticed here. 



Order I. Stylommatophora. 



The Pulmonates included in this order have the eyes placed at 

 the extremities of long feelers, which are retractile and capable of 

 invagination. The shell may be rudimentary or even absent, but 

 is usually well developed. The following are, from a palasonto- 

 logical point of view, the most important families included in this 

 order : — 



Family i. Testacellid^e. — This family includes carnivorous 

 Pulmonates, with a spiral shell of very variable size. In Testacella 

 itself, the animal is slug-like, and there is a minute ear-shaped shell 

 placed at the hinder end of the body. Fossil forms of this genus 

 have been found in the later Tertiary deposits. 



In Glandifia, on the other hand, there is a well-developed spiral 

 shell, within which the body of the animal can be withdrawn. The 

 recent types of this genus inhabit warm regions, and the fossil forms 

 date from the Upper Cretaceous period. 



Family 2. LimacidjE. — In this family are comprised the "Slugs," 

 in which there is a minute rudimentary shell concealed within the 

 mantle. Species of Limax have been recognised as occurring in the 

 late Tertiary and Quaternary deposits. 



Family 3. Helicid^e. — This family includes the ordinary " Land- 

 snails," in which there is a well-developed shell, capable of contain- 

 ing the entire animal. A large number of fossil representatives of 

 this family are known, chiefly belonging to the Tertiary period, but 



