138 THE STORY OF THE EARTH AND MAN. 



ages. We may connect this with other laws in the 

 sequel. 



In the coal measures we also meet, for the first 

 time in our ascending progress, the land snails so 

 familiar now in every part of the world, and which 

 are represented by two little species found in the coal 

 formation of Nova Scotia. The figures of these must 

 speak for themselves ; but the fact of their occurrence 

 here and the mode of their preservation require some 

 detailed mention. The great province of the Mollusks 

 we have carried with us since we met with the Lingulae 

 in the Primordial, but all its members have been 

 aquatic, and probably marine. For the first time, in 

 the Carboniferous period, snails emerge from the 

 waters, and walk upon the ground and breathe air; 

 for, like the modern land snails, these creatures no 

 doubt had air- sacks instead of gills. They come 

 suddenly upon us two species at once, and these 

 representing two distinct forms of the snail tribe, the 

 elongated and the rounded. They were very numer- 

 ous. In the beds where they occur, probably 

 thousands of specimens, more or less perfect, could 

 be collected. Were they the first-born of land snails ? 

 It would be rash to affirm this, more especially since 

 in all the coal-fields of the world no specimens have 

 been found except at one locality in Nova Scotia ;* and 

 in all the succeeding beds we meet with no more till 

 we have reached a comparatively modern time. Yet 



* Bradley has recently announced the discovery of other 

 species in the coal-field of Illinois 



