98 Beautiful Shells. 
be so perfect and admirable, that eighty pounds 
was afterwards offered for it, and refused. 
These Naide have not a bissus like the Marine 
Mussels, they are therefore never attached to one 
object ; they use their foot as a propeller in tra- 
versing the muddy floor of the pond or river, and 
they have a very funny way of getting along indeed ; 
first, they opcn the valves of the shell, put out the 
foot, and, after some little hard work, manage to 
set themselves up on edge ; they then proceed by a 
series of jerks, leaving a deepish furrow in the mud 
behind them. 
We will now go to Fig. 3, the Spined Cytherea, 
the Cytherea, or Venus Dione of naturalists; the 
meaning of the term is the mother of Venus, who 
was, aS you will remember, the goddess of beauty, 
given to this shell, perhaps, because it is entitled to 
occupy a place at the head of the Cytherea, a genus 
of the Cardiide, or Cockle family, of which genus 
there are about seventy-eight living species ; this, 
as it is the most rare, is also, perhaps, the most 
beautiful ; it is found in the seas of America, and is 
remarkable for the row of spines on the hinder 
border of each valve; these vary much in size and 
number, being in some individuals long and far 
apart, in others, short, thick, and closely set. ‘The 
