32 HALF-HOURS IN THE GREEN LANES. 



of remains of pike, perch, &c., in deposits as old as 

 those containing evidence of man's first occupancy 

 of the earth, prove what a great antiquity, in point 

 of actual time, is possessed by our fishes and mollusca, 

 and tell plainly of the great changes that have taken 

 place since they overspread these areas, after the 

 gradual toning down of the rigorous climate of the 

 Glacial epoch. 



Passing now to another group of objects of a 

 lowlier kind, but not less differentiated and adapted 



Fig. 19. 



Teeth of Paludma vivipara. 



to an aquatic life, we come to the fresh-water 

 molluscs. The student is sure to find species in 

 every streamlet and tarn. In the latter, especially, 

 underneath the vegetation floating on the water, or 

 at the bases of the sedges, rushes, and grasses which 

 fringe the sides, a careful examination will dis- 

 cover an abundant harvest. The Fresh- water Snails 

 (Lymnea stagnates) creep about, the epidermis of 

 their shells covered with a green scum that affords 

 most interesting material to the microscopist. 

 Other Lymneas pereger, auricularia, glaber, &c. 



