14 EAMBLES IN SEARCH OF SHELLS. 



the same individual. In the latter class, however, an 

 individual requires to be fertilized by another of its 

 own species. But, as observed by Leach, the sexual 

 parts are so far situated from each other, that one 

 individual is able to perform the functions of each 

 sex with two others at the same time, and it is 

 consequently not uncommon to find several of these 

 molluscs engaged at once in a mutual interchange of 

 attentions. 



Having thus changed, the shape of our model, let 

 us now reduce the size of it, increase its elongation, 

 and narrow the width until the shell becomes oblong, 

 with the whorls drawn out and spire long. We 

 then get some idea of another genus, Bulimus, of 

 which there are three British species, all herbivorous 

 acutus, the commonest, inhabiting downs and sand- 

 hills ; montanus (or lackhamensis) and obscurus, 

 both of which are found on the trunks of trees and 

 amongst dead leaves in woods and hedgerows. 



By cutting off a couple of whorls or so from the 

 apex, and rounding it, at the same time compressing 

 the model so as to make the whorls more compact, 

 we get some notion of Pupa. The four species in 

 this genus secale, ring ens, umbilicata, and margin- 



