THE FISHES, MOLLUSCA, AND OTHER OBJECTS. 39 



important part they have played in the economy of 

 our planet. All of them have a geological antiquity 

 far transcending that of the human race ! The 

 anodons have been in existence since the Devonian 

 period, and the fossil specimens from the upper 

 Devonian green sandstones of Kilkenny are seen to 

 possess all the characters which still distinguish the 

 genus. It would seem that when certain classes of 

 animals have attained the characters best suited to 

 them, evolution is arrested, and the form becomes 

 stereotyped. Paludinas are so abundant in the 

 upper Secondary fresh- water strata, especially those 

 of the Wealden, that they form thick beds of lime- 

 stone. The well known " Purbeck marble," masses 

 of which you might have noticed forming the font 

 of the village church hard by, is wholly composed of 

 the shells of extinct Paludinae ! The Planorbis is a 

 familiar fossil form in the upper Eocene beds of Hamp- 

 shire, where it may be seen of a size that well suited 

 the sub-tropical conditions of climate which marked 

 the period when it was alive. Cyclas, Pisidium, 

 Lymnea, &c., are equally abundant in beds of the same 

 age. Even the existing species have a vast antiquity. 

 Planorbis corneas, P. vortex, and Valvata piscinalis, are 

 found fossil in the Norwich crag, showing they were 

 living in England just before the advent of the long 

 northern winter of the " Glacial epoch," when Europe 

 was swathed in an ice-sheet as Greenland is now ! * 



* The student desirous of making himself more fully acquainted 

 with our land and fresh-water molluscs, cannot do better than get 

 Tflte's work on this subject, published by Hardwicke, Piccadilly 



