10 EAMBLES IN SEARCH OF SHELLS. 



in Siberia and the northern parts of Asia and 

 America. Of these we may have occasion to 

 speak later. Meantime it will be desirable to 

 get a general idea of some of the commoner forms 

 of shells, and the subject is at once simplified by 

 dividing the whole number of species into two 

 classes — the Bivalves (Conchifera) , of which the 

 common mussel furnishes an illustration, and the 

 Univalves {Gasteropoda), of which the garden snail 

 is a familiar example. 



Amongst the aquatic shells there are both bivalves 

 and univalves, but with the terrestrial species only 

 univalves occur. The reason for this is to be found 

 in the difference of structure which exists in the 

 animal itself, particularly in the organs of respira- 

 tion, and which enables one species to breathe and 

 live where another would assuredly die. 



The bivalves, which are all LamelUhranchiata, — 

 that is, having leaf- like gills, — may be separated into 

 three very distinct and well-marked families — namely, 

 Sphceriidce (from the spherical shape of the species) ; 

 Unionidce, which contains the mussels (from unio, 

 a pear], one of them being a pearl-bearing species), 

 and Dreissenidw (so named after M. Dreissens, a 



