THE OLDEST AIR-BREATHERS 289 



divisions of the group of air-breathing snails (Pulmonifera), the 

 Operculate, or those with a movable plate to close the mouth 

 of the shell, and the Inoperculate, or those that are destitute 

 of any such shelly lid or operculum to close the shell, the first 

 has been traced no farther back than the Eocene. The second 

 or inoperculate division, includes some genera that are aquatic 

 and some that are terrestrial. Of the aquatic genera no re- 

 presentatives are known in formations older than the Wealden 

 and Purbeck, and these only in Europe. The terrestrial group, 

 or the family of the Helicidcz, which, singularly enough, is that 

 which diverges farthest from the ordinary gill-bearing Gastero- 

 pods, is the one which has been traced farthest back, and 

 includes the Palaeozoic species. It is further remarkable that 

 a very great gap exists in the geological history of this family. 

 No species are known between the Carboniferous and the early 

 Tertiary, though in the intervening formations there are many 

 fresh-water and estuarine deposits in which such remains 

 might be expected to occur. There is perhaps no reason to 

 doubt the continuance of the Helicidse through this long por- 

 tion of geological time, though it is probable that during the 

 interval the family did not increase much in the numbers of 

 its species, more especially as it seems certain that it has its 

 culmination in the modern period, where it is represented by 

 very many and large species, which are dispersed over nearly 

 all parts of our continents. 



The mode of occurrence of the Palaeozoic Pulmonifera in 

 the few localities where they have been found is characteristic. 

 The earliest known species, Pupa vetusta, was found, as 

 already stated, in the material filling the once hollow stem of 

 a Sigillaria at the South Joggins in Nova Scotia, and many 

 additional specimens have subsequently been obtained from 

 similar repositories in the same locality, where they are associ- 

 ated with bones of Batrachians and remains of Millipedes. 

 Other specimens, and also the species Zonites friscus, have 



