CLASSIFICATION 27 
are enclosed between the two, right and left, lobes of 
the mantle, which in their turn are covered by the 
two shelly plates, or valves. In a few instances the 
mantle is extended and reflected over the valves, and 
partly (Galeommidz) or completely (Ephippodonta, 
etc.) envelops them. 
The mantle lobes are attached along the back and 
extend out to the margins of the valves. In the 
more primitive forms the mantle margins are quite 
simple, and open from the front round the ventral 
edge to the back; but many different modifications 
take place in the higher forms. Instead of the 
margins being simple, there may be folds, thicken- 
ings, protuberances, tentacles (Plate XV., Fig. 5), 
various glands, and even eyes (Plate XV., Fig. 8). 
Nor are the margins always free; indeed, in extreme 
cases they are united nearly the whole way round. 
There are various stages (Plate IV., Fig. 4, A-F): 
reckoning the open one as the first (A), then in the 
second (B), instead of the water being admitted all 
the way round, the edges of the mantle are kept 
closely applied to each other except where the foot 
protrudes and at two points at the hinder end, one 
of which serves to admit the fresh water to the gills, 
and is called the ‘“‘inhalent aperture,” whilst the 
other allows the fouled water to escape, and is 
termed the “‘exhalent aperture.” In the next stage 
(C), the edges of the mantle are united permanently 
at the point between these two openings; and in the 
